x/tools/gopls: support LocationLink for relevant requests

[aktau]

When requesting textDocument/definition gopls returns a Location.

This FR is about supporting returning a LocationLink instead for clients that support it (a client capability).

Rationale: some editors/plugins can use this to support previewing the entire function on hover. See neovim/neovim#12218 and neovim/neovim#12368 for an in-depth description.

In terms of other language servers that do this, it looks like rust-analyzer does it as well.

All (current) LSP actions that can return LocationLink's:

• textDocument/declaration
• textDocument/definition
• textDocument/typeDefinition