Fix completion-filtering by checking actual word boundaries & reduce Ruby word chars

[mrnugget]

This fixes #9069 by

1. reverting Ruby: Code Completion not working  #7819
2. fixing completion filtering with regards to word boudaries

For (2) see explanation in commit message:

Previously, this would only split words on upper-lower boundaries or
on _/-.

The result was that we would filter out completions too aggressively.
The filter works by taking a suggested completion, say foo_bar_lol, split
it up into words - foo, bar, lol- and check whether any of the words start with the same characters as what the user already typed:fo, or bar`,
...

In the case of Ruby, though, : wasn't considered a word boundary. If the
LSP would return :foobar when the user typed :foo, we'd check if there are
any completions that match foo (because that's the current word) but
we'd compare against foobar, not : or :foobar.

With this change, we get more match candidates and thus more completions in Ruby.

With that we can do (1) because we don't need these characters as word characters anymore to trigger completions.

Release Notes:

• Fixed word boundaries in Ruby by restoring old behavior (@, :, ... are no longer considered word characters) (#9069)
• Fixed completions being filtered out when they happened at word boundaries on special characters (e.g. :)