Allow excluding composed keys like <space> and <escape> from sites

There's many issues which touch on this, but I'll try to bring the discussion together here.

The ESC key is used in Vimium to exit insert mode, find mode, link hints, etc.

Many sites also use ESC for similar purposes. Vimium prevents the site from receiving the ESC keypress.

#1985 introduced a passNextKey command, so that you can pass through a one-off escape keypress to the site.

Many issues have asked for the opposite: disable Vimium's ESC key on specific sites, and instead map the vimium escape functionality to another key.

This is not possible because we only allow single-letter keys to be excluded, and we do not support unmap <escape>.

A solution is to allow excluding and unmapping composed key strings like <escape> and <space>. This is viable, but does change the exclusions UI a bit and require a preferences data migration.

Some related issues:

• #3022
• #3203
• #3948 and (@gdh1995's comment)

Start by tracing the exclusions UI, key handling, and preferences data migration described in the issue, along with the related issues #3022, #3203, and #3948. Done means composed keys such as and can be excluded and unmapped for selected sites without breaking existing preferences.