Expand focus commands

[KSR-Yasuda]

Added commands in focus mode:

• s : split window
• v : vsplit window
• 1 - 9 : Move to window 1 - 9
• $ : Move to the last window
• w : Move to the next window
• W : Move to the previous window

[simeji]

Thank you for this! Being able to split, vsplit, and jump to windows by number without leaving the plugin is a great quality-of-life improvement for focus mode.

I made two small changes before merging:

• Removed the unused num0 entry from s:codeList (no command was mapped to it in any mode)
• Added the new keys to the README options table