shell: rewrite fish plugin

[kidonng]

Years ago in #671 I did a big refactor to the fish plugin, however I did not understand the code correctly. It removed a feature and has a minor bug (#928).

This pull request rewrites the fish plugin. It is backwards compatible (still binds to Ctrl G, user key binding to the main function _navi_smart_replace keeps working). Current behavior:

1. Retrieve current command line as query to navi
2. Check if there's a best match and replace current command line
3. Open navi with current command line as query, replace current command line if user makes a selection. Previously the plugin only opens navi if command line is empty, i.e. if query doesn't have a best match the key binding will do nothing.