feat: Add source/action to select and execute vim-gin's actions

[mityu]

This PR adds a source to get a list of gin actions from the current gin buffer, and an action to execute a selected action on the current gin buffer.

This is example screenshot of gin-action picker invoked on the gin-log buffer in this PR's branch.

And configuration example:

import { ginAction } from "@vim-fall/extra/source/gin-action";
import { defaultGinActionExecuteActions } from "@vim-fall/extra/action/gin-action-execute";

definePickerFromSource("gin-action", ginAction, {
  previewers: [],
  actions: {
    ...defaultGinActionExecuteActions,
  },
  defaultAction: "gin-action-execute",
});

---

Note that in the current implementation, the gin-action-execute doesn't support selectedItems parameter. The reason is that we don't know which order we should invoke actions when multiple actions are given because the order of git operations often matter.
If you have another idea or advices, please let me know.

[mityu]

I came up with the idea that the behavior/limitations of this source should follow those of the built-in action selector's. I'll check it.

[mityu]

It seems that the default action picker just ignores the selectedItems property, so I think it's ok that gin-action-execute only checks item property.

• The place where getting action to execute from result of action picker.
  • https://github.com/vim-fall/fall.vim/blob/9464beb9bd652f8f978b51a8ce43f3d93920e154/denops/fall/main/picker.ts#L307-L329
• The place where action picker returns its result.
  • https://github.com/vim-fall/fall.vim/blob/9464beb9bd652f8f978b51a8ce43f3d93920e154/denops/fall/picker.ts#L416-L426