feat(pack): add code actions in confirmation buffer

Problem: No way to granularly operate on plugins when inside
confirmation buffer.

Solution: Implement code actions for in-process LSP that act on "plugin
at cursor":
  - Update (if has updates).
  - Skip updating (if has updates).
  - Delete.

  Activate via default `gra` or `vim.lsp.buf.code_action()`.

runtime/doc/pack.txt

@@ -402,6 +402,9 @@ update({names}, {opts})                                    *vim.pack.update()*
         • 'textDocument/hover' (`K` via |lsp-defaults| or
           |vim.lsp.buf.hover()|) - show more information at cursor. Like
           details of particular pending change or newer tag.
+        • 'textDocument/codeAction' (`gra` via |lsp-defaults| or
+          |vim.lsp.buf.code_action()|) - show code actions available for
+          "plugin at cursor". Like "delete", "update", or "skip updating".
         Execute |:write| to confirm update, execute |:quit| to discard the
         update.
       • If `true`, make updates right away.