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/xlang.test.ts

@@ -1,12 +1,12 @@
 import * as fs from "fs";
 import * as path from "path";
 import * as YAML from "yaml";
-import {convertWorkflowTemplate} from "./model/convert";
-import {NoOperationTraceWriter} from "./templates/trace-writer";
-import {File} from "./workflows/file";
-import {FileProvider} from "./workflows/file-provider";
-import {fileIdentifier, FileReference} from "./workflows/file-reference";
-import {parseWorkflow} from "./workflows/workflow-parser";
+import {convertWorkflowTemplate} from "./model/convert.js";
+import {NoOperationTraceWriter} from "./templates/trace-writer.js";
+import {File} from "./workflows/file.js";
+import {FileProvider} from "./workflows/file-provider.js";
+import {fileIdentifier, FileReference} from "./workflows/file-reference.js";
+import {parseWorkflow} from "./workflows/workflow-parser.js";
 
 interface TestOptions {
   "include-source"?: boolean;