ESM migration: Add .js extensions for node16 moduleResolution (#257)

Migrate expressions, workflow-parser, and languageservice packages to use
proper ESM imports with .js extensions that work with node16 moduleResolution.

Changes:
- Update tsconfig.build.json in each package to use module: node16 and
  moduleResolution: node16
- Add .js extensions to all relative import paths (Option B approach)
- Fix yaml internal type imports in workflow-parser by defining local types
- Add skipLibCheck to handle @types/node compatibility issues
- Add TypeScript 5.8.3 override in root package.json
- Add ESM migration plan documentation

The languageserver package is deferred due to test hang issues that need
further investigation.

Related #154 - Upgrade moduleResolution from node to node16 or nodenext
Related #110 - Published ESM code has imports without file extensions
Related #64 - expressions: ERR_MODULE_NOT_FOUND attempting to run example
Related #146 - Can not import @actions/workflow-parser

Test results:
- expressions: 1068 tests passed
- workflow-parser: 292 tests passed
- languageservice: 452 tests passed

* docs: update ESM migration plan with findings

- Update languageserver blocker: vscode-languageserver v8.0.2 lacks ESM
  exports (not a test hang issue)
- Document that Option B (manual .js extensions) was chosen over Option A
  due to ts-jest compatibility issues
- Add workaround for yaml package internal types (LinePos, NodeBase)
- Update migration status table with accurate reason for deferral
- Add skipLibCheck note for @types/node compatibility

languageservice/src/expression-hover/expression-pos.test.ts

@@ -1,9 +1,9 @@
 import {parseWorkflow} from "@actions/workflow-parser";
 import {File} from "@actions/workflow-parser/workflows/file";
-import {nullTrace} from "../nulltrace";
-import {getPositionFromCursor} from "../test-utils/cursor-position";
-import {findToken} from "../utils/find-token";
-import {ExpressionPos, mapToExpressionPos} from "./expression-pos";
+import {nullTrace} from "../nulltrace.js";
+import {getPositionFromCursor} from "../test-utils/cursor-position.js";
+import {findToken} from "../utils/find-token.js";
+import {ExpressionPos, mapToExpressionPos} from "./expression-pos.js";
 
 describe("mapToExpressionPos", () => {
   it("simple expression", () => {

languageservice/src/expression-hover/expression-pos.ts

@@ -3,8 +3,8 @@ import {ensureStatusFunction} from "@actions/workflow-parser/model/converter/if-
 import {TemplateToken} from "@actions/workflow-parser/templates/tokens/template-token";
 import {isBasicExpression, isString} from "@actions/workflow-parser/templates/tokens/type-guards";
 import {Position, Range as LSPRange} from "vscode-languageserver-textdocument";
-import {mapRange} from "../utils/range";
-import {posWithinRange} from "./pos-range";
+import {mapRange} from "../utils/range.js";
+import {posWithinRange} from "./pos-range.js";
 
 export type ExpressionPos = {
   /** The expression that includes the position */

languageservice/src/expression-hover/visitor.test.ts

@@ -2,13 +2,13 @@ import {data, DescriptionDictionary, Lexer, Parser} from "@actions/expressions";
 import {convertWorkflowTemplate, parseWorkflow} from "@actions/workflow-parser";
 import {ErrorPolicy} from "@actions/workflow-parser/model/convert";
 import {File} from "@actions/workflow-parser/workflows/file";
-import {ContextProviderConfig} from "../context-providers/config";
-import {getContext, Mode} from "../context-providers/default";
-import {getWorkflowContext} from "../context/workflow-context";
-import {validatorFunctions} from "../expression-validation/functions";
-import {nullTrace} from "../nulltrace";
-import {getPositionFromCursor} from "../test-utils/cursor-position";
-import {HoverVisitor} from "./visitor";
+import {ContextProviderConfig} from "../context-providers/config.js";
+import {getContext, Mode} from "../context-providers/default.js";
+import {getWorkflowContext} from "../context/workflow-context.js";
+import {validatorFunctions} from "../expression-validation/functions.js";
+import {nullTrace} from "../nulltrace.js";
+import {getPositionFromCursor} from "../test-utils/cursor-position.js";
+import {HoverVisitor} from "./visitor.js";
 
 const contextProviderConfig: ContextProviderConfig = {
   getContext: (context: string) => {

languageservice/src/expression-hover/visitor.ts

@@ -13,7 +13,7 @@ import {
 } from "@actions/expressions/ast";
 import {FunctionDefinition} from "@actions/expressions/funcs/info";
 import {Pos, Range} from "@actions/expressions/lexer";
-import {posWithinRange} from "./pos-range";
+import {posWithinRange} from "./pos-range.js";
 
 export type HoverResult =
   | undefined