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

[mityu]

This is vim-fern version of #1 .

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

Example

This is an example screenshot of fern-action picker works at the root directory of this repository.

The configuration is like this.

import { fernAction } from "@vim-fall/extra/source/fern-action";
import { defaultFernActionExecuteActions } from "@vim-fall/extra/action/fern-action-execute";

definePickerFromSource("fern-action", fernAction, {
  previewers: [],
  actions: {
    ...defaultFernActionExecuteActions,
  },
  defaultAction: "fern-action-execute",
});

---

Note that as same as gin-action, the fern-action-execute action ignores the selectedItems property and only respect the item property.
Ref: #1 (comment)