feat: simplify and enhance prompt weight splitting

[nderscore]

• Simplify split_weighted_subprompts method by replacing most of the logic with single a regular expression
• The original syntax is still fully supported, but has been expanded to handle all of the following variations of the syntax:
  • foo:2 integers
  • foo:-1.23 decimals and negative numbers
  • foo: no weight (defaults to 1)
  • foo::123 midjourney-style double-colon syntax
  • foo:2bar:1 foo:bar missing space characters between subprompts? no problem
  • foo:2 bar:1 spaces are fine too though
  • foo:    bar foo:1.2    bar:3 multiple spaces as well
  • foo:2 bar and the last subprompt doesn't need a colon
  • foo and a plain prompt works still too
• (Edit:) Added support for escaping colon characters with a backslash
  • When escaped, actual colon characters are inserted into prompt tokens
  • foo\:bar -> foo:bar unsplit
• Add a guard against normalization code breaking when prompt weights add up to 0
• Move weight normalization code into the split_weighted_subprompts method as an argument

[lstein]

I tested this out with a variety of weighting formats and it seems to be performing as advertised. I also compared images from this version to the previous weighted prompt implementation, and they were identical. Thank you!