Implemented find char with leap/easyMotion/flash style

[yerlaser]

Please note this is not a replacement for goto_word - that is still the fastest way to navigate within the document window.

However, I was always frustrated by how primitive f/F/t/F family of commands are to select portions of line(s).
This PR allows me to fully replace f/F/t/T in my personal keybindings for a much faster and precise selections within the current or adjacent lines.

Some more details:

1. When there are multiple selections/cursors the commands bail out to their built-in counterparts
2. The jump labels should not clash with characters next to the searched one. This allows to narrow the search by adding additional search characters
3. Pressing Enter moves/extends to end of line similar to what built-in commands do
4. I don't use and test the non-extend variants and include them here only for completeness
5. This PR adds the following commands: flash_forward, extend_flash_forward, flash_backward, extend_flash_backward, flash_forward_till, extend_flash_forward_till, flash_backward_till, extend_flash_backward_till

Overall, the code is highly unoptimized and surely contains bugs, hence making it a draft.
And I can understand that the chances of its inclusion are low.
But it's still quite useful for me and I thought it could benefit others, too.

And last but not least many thanks to @shybovycha for the original implementation in this (now rejected) PR: #14644

[CalebLarsen]

This feels like it is still in the spirit of the previous PR which was rejected. I think that this should belong in a plugin, not in helix proper.

[yerlaser]

I posted it because it might be useful for some.
But, the PR is not ready yet.
I want to reuse as much existing code as possible so it will become a variant of built-in commands.