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

workflow-parser/src/workflows/yaml-object-reader.ts

@@ -11,10 +11,8 @@ import {
   Scalar
 } from "yaml";
 import type {Document} from "yaml";
-import type {LinePos} from "yaml/dist/errors";
-import type {NodeBase} from "yaml/dist/nodes/Node";
-import {ObjectReader} from "../templates/object-reader";
-import {EventType, ParseEvent} from "../templates/parse-event";
+import {ObjectReader} from "../templates/object-reader.js";
+import {EventType, ParseEvent} from "../templates/parse-event.js";
 import {
   BooleanToken,
   LiteralToken,
@@ -23,8 +21,12 @@ import {
   NumberToken,
   SequenceToken,
   StringToken
-} from "../templates/tokens/index";
-import {Position, TokenRange} from "../templates/tokens/token-range";
+} from "../templates/tokens/index.js";
+import {Position, TokenRange} from "../templates/tokens/token-range.js";
+
+// Internal yaml types - defined locally since yaml package doesn't export them
+type LinePos = {line: number; col: number};
+type NodeBase = {range?: [number, number, number] | null};
 
 export type YamlError = {
   message: string;