fix(lsp): complete npm package exports (#34675)

Adds npm package export subpath completion for import specifiers by
reading concrete export keys from npm package metadata and caching them
in the LSP npm search API.

Adds unit coverage for npm export completion/metadata extraction and an
LSP integration regression for completing a non-index npm export route.

Closes #26872
Closes denoland/divybot#400

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