Implement more special Vim registers

[xPMo]

For each register:

1. Should this be implemented in Zsh?
2. Should this be synced between Zsh and the editor? Also, if RO-registers are not synced, should these still be RO?
3. Should these be automatically filled/generated?

Currently being developed in dev.

• [x] [RO] ": - Command-line register Wontfix, : conflicts with the new zstyle interface.
• [x] [RO] ". - Last insert register - unsynced read-only, must be enabled by adding track-insert function to zle-keymap-select widget.
• [x] [RO] "% - Current file register - Gets full path of current file from vim/nvim.
• [x] "- - Small delete register - Synced from editor if the style is set as such.
• [x] "# - Alternate file register - unsynced read-only, insets next entry in directory stack Gets full path of alternate file from vim/nvim
• [x] "= - Expression register - Synced from editor if the style is set as such.
• [x] "_ - Black hole register - Implemented in zsh
• [x] "/ - Last search register - implemented in fa19392: Does not sync, read-only. Uses Zsh-native LASTSEARCH variable.