Move implementation to standalone forgit

Previously we had two different forgit implementations: one for zsh/bash
and one for fish. In this commit we move the bash implementation of the
forgit functions to the git-forgit script in order to have them in the
form of an executable script. This makes using forgit possible without
any shell plugin at all and furthermore removes the need to maintain a
separate implementation for the fish shell.

The simplest way of using forgit from now on is to put git-forgit
somewhere in your PATH and call it via "git forgit", without the need
for any other files.

We keep the shell plugins, though, and maintain backwards compatibility
by retaining the plugin functions as simple wrappers around the
git-forgit script, making the plugins mainly a collection of aliases
only.

.github/workflows/ci.yaml

@@ -41,7 +41,7 @@ jobs:
         fish --version; echo
 
     - name: Shellcheck
-      run: shellcheck forgit.plugin.sh
+      run: shellcheck forgit.plugin.sh bin/git-forgit
 
     - name: Test bash
       run: bash forgit.plugin.sh