textDocument/declaration

[sam-mccall]

LSP now supports textDocument/declaration and textDocument/definition separately.

Proposed behavior:

• for declaration, we return the best decl we have
• for definition, we return the definition if we have one, else best decl
• if we want a generic "go to" that returns both, then add an extension

Caveats: this breaks back-compat (decl no longer returned to old clients). No way to detect these via capabilities.

[sam-mccall]

There's no server capability in LSP but I think that's just a protocol bug: microsoft/language-server-protocol#673

We should advertise it anyway.

[sam-mccall]

Fixed in r352864