fix(doc): handle non-ASCII doc lint diagnostics (#34626)

## Summary
- render doc lint diagnostic highlights using UTF-8 character boundaries
- cover non-ASCII exported symbols and private type references in doc
lint specs

## Testing
- cargo fmt --check
- git diff --check
- cargo check -p deno --bin deno

Note: attempted to run the focused `deno doc --lint` fixture, but
building a runnable binary required a separate full codegen pass; the
type check completed successfully.

Closes denoland/divybot#366

Co-authored-by: divybot <divybot@users.noreply.github.com>
Co-authored-by: Divy Srivastava <me@littledivy.com>