Convert TypeScript sources to NodeNext ESM

[dibarbet]

Summary

Replacement for #9617, rebuilt on the dev/esm-tooling-foundation replacement layer (#9631) so the dev-branch stack receives the intended CI coverage.

• set the package to ESM and upgrade TypeScript to 5.7.3
• compile the main repository with NodeNext/NodeNext, allowJs, and rewriteRelativeImportExtensions, using normal tsc emit
• inherit Jest project-name support-file emission from Prepare tooling for repository-wide ESM #9631: tsc emits test/jestProjectNames.mjs directly, with no separate copy task
• use explicit .ts relative source specifiers (1,448 after deleting the copy task and its sole .ts import) plus six intentional .mjs imports, which emit as runtime .js/.mjs imports
• keep the ESM bundle's createRequire interop while removing the temporary __dirname/__filename shim and retaining targeted CommonJS compatibility fixes
• keep integration Jest transformation isolated without a Preserve/Bundler split build configuration

Validation

• targeted ESLint --fix plus npm run compileDev
• compiler-owned out/test/jestProjectNames.mjs exists and exports values semantically identical to the source; zero out/**/jest.config.mjs
• emitted JavaScript audit: zero relative .ts imports; all four integration runners import runIntegrationTests.js and jestProjectNames.mjs
• raw Node import of all four emitted integration runners
• npm run packageDev and node --check dist/extension.mjs
• npm run test:unit (514 passed)
• npm run omnisharptest:unit (401 passed)
• npm run test:integration:untrusted (1 passed)
• npm run test:integration:csharp (73 passed, 3 skipped)
• git diff --check

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[dibarbet]

Superseded by #9635, which consolidates the complete native ESM migration into one final-state commit and removes intermediate workaround history.