*: Move third-party plugins into .local/share/

Move all submodules that were under .config/ into .local/share/ (alias
XDG_DATA_HOME). I got convinced that they feel out of place in .config/.

Instead of updating the paths from which they are sourced (e.g.
`TMUX_PLUGIN_MANAGER_PATH` or `packpath`), keep a symlink in the old
location that points to the new one. This has the benefit that
everything related to one program remains mostly accessible from the
same place instead of spreading everything out.

I am still unsure if I really like the new location as I affiliate user
**data** more with something that has to be backed up as it is
non-recreatable. And moving them out of .config/ had the reasoning that
they are not my config files but rather data from the internet that
could be retrieved when deleting. But using XDG_CACHE_HOME seems
unreasonable as well.
We'll see.

Move the fzf-repo from ./local/bin/ to .local/share/.

Update all relative symlinks' (e.g. fzf) targets.

.local/share/zsh/plugins/fzf

@@ -0,0 +1 @@
+../../fzf-repo/