Support for code action for diagnostics is missing

[fhs]

LSP provides a way of applying fix for diagnostics automatically. User selects a suggested fix (code action) for a diagnostic, and then LSP server sends the edits we need to apply to fix it.

Right now we manually apply the source.organizeImports code action (goimports for Go) every time we format a file, but we should support other code actions triggered from a diagnostic (e.g. quickfix).

Main challenge of implementing this would be figuring out a good UI.