feat: add variable segment movements and textobject

[gertqin]

Hi,

I only discovered helix a few days ago, but I've already (more or less) switched over from nvim - so thanks for an awesome editor! 🙂

However, one plugin I really like in nvim is vim-textobj-variable-segment which makes it a lot easier to navigate variables in title/camel/snake-case and edit parts of it.

In this draft PR I have made a simple implementation of it in helix, i.e. added

• move/extend_prev_partial_word_start (keybind H)
  • Ex let |VarInTitleCase => let |Var|InTitleCase => let Var|In|TitleCase
• move/extend_next_partial_word_end (keybind L)
  • Ex let var_in_snake_case| => let var_in_snake|_case| => let var_in|_snake|_case
• textobject partial word (keybind v in match inside/outside mode)
  • Ex let var_in_sn|ake_case => mav => let var_in|_snake|_case
  • Ex let var_in_sn|ake_case => miv => let var_in_|snake|_case

(I named everything partial_word in the code, because that's what I thought the vim plugin was called until I looked it up...)

Do you think these commands belong to the helix core? If that's the case then I'll add unit tests to the PR.

I'll also gladly rename partial_word to something better if you can think of something, and you probably also want to reserve the keys H and L for something else.. (I think the movements could be added without default keybindings).

[kirawi]

Duplicate of #8147 (leave this open for the maintainers to see which solution is preferred)