Modernize

- Remove obsolete dependencies.
  - abort-controller; `AbortController` is stable since Node 15.4.0.
  - source-map-support; `--enable-source-maps` is supported since Node
    12.12.0.
- Update dependencies.
- Enable strict mode in `tsconfig.json`.
- Use `which(..., {nothrow: true})` and distinguish between
  binary-not-in-PATH and binary-access-failed. Log an error.
- Check for presence of clangd binary in zip archive before extracting.
- Use native promises APIs instead of `promisify`.
- Assign severity to all logs (no more `console.log`).