Problem: In `vim.pack.update()` confirmation buffer it might be useful
to be able to use `gx` (open link at cursor) when cursor is on
something like commit or tag.
Solution: Add `textDocument/documentLink` method support for the
in-process LSP. This may be used by LSP clients and makes `gx`
automatically work.
The shortcoming is that this requires tracking how to construct a URL
from source and commit/tag. Currently only GitHub hosted repositories
are supported.
d01dc690e1
