Add bindings for K to open help window (or a hover window) for all the syntax constructs

[novusnota]

Depends on #7.

See how they did it:

• https://github.com/tpope/vim-scriptease
• https://github.com/tpope/vim-rails
• https://github.com/vim-ruby/vim-ruby
• https://github.com/fatih/vim-go

This can also try to use tact doc (once it arrives) to add additional documentation for existing things or tact language-server for hover window (integrate with vim-lsp plugin).