[LevK]: Build and test as a standalone package

Split out of actions/toolkit (packages/http-client, history kept by filter-repo).
Brings the 2024 [LevK] packaging edits forward onto current upstream:

- tsconfig.json: the monorepo root's compiler options inlined, no extends
- jest.config.cjs: the root's ts-jest transform, rooted at tests/
  (isolatedModules: ESM lib/ and sources run through CommonJS)
- __tests__/ -> tests/
- package.json: test = jest, build = tsc; jest/ts-jest/typescript as
  devDependencies; repository, homepage, bugs on git.levk.dev
- .gitignore: node_modules/, lib/, audit.json
- HttpCodes.PayloadTooLarge = 413 (the 2024 edit; still missing upstream)

Not carried over from 2024: the _getExistingOrDefaultHeader type
narrowing (upstream has since fixed it properly), version reset to 1.0.0 (dependents' ranges
need upstream's numbering) and the Unlicense field (the code is MIT).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UHXzGrsxfkGk8LmwrBbHNZ

.gitignore

@@ -1,2 +1,6 @@
 testoutput.txt
 npm-debug.log
+
+lib/
+audit.json
+node_modules/