feat(lsp): jsr support first pass

[nayeemrmn]

This implementation heavily depends on there being a lockfile, meaning JSR specifiers will always diagnose as uncached unless it's there. In practice this affects cases where a deno.json isn't being used. Our NPM specifier support isn't subject to this.

The reason for this is that the version constraint solving code is currently buried in deno_graph and not usable from the LSP, so the only way to reuse that logic is the solved-version map in the lockfile's packages.specifiers.

TODO: Non-erroneous tests for caching and type-checking.