Refactor: Replace deferred code used for fzf preview with functions

Removes the deferred code that is used for creating the fzf preview functions and replaces it with _forgit_*_preview functions instead. These functions are exposed as forgit commands so they can be invoked from the fzf subshell. We split the exposed commands into public_commands and private_commands. The only difference between them is that public_commands are mentioned in the help text.
This commit changes the flags variable in _forgit_blame from a string to an array. This is necessary to allow the flags to be passed to _forgit_blame_preview as individual arguments.