DRAFT: Better white space collapse

[nicoburns]

Implements the white-space-collapse portion of:

• Full support for the CSS white-space property #619

Generated with Opus 4.8 High, with review/revisions from Fable. Some manual review has been done. More coming soon (before this is moved out of draft).

Changes made

• Add PreserveBreaks, PreserveSpaces, and BreakSpaces variants to WhiteSpaceCollapse.
• Implement the actual whitespace collapsing behaviour for these modes in the TreeBuilder
  (+ optimize the whitespace-collapsing in the TreeBuilder to reuse a single buffer in-place rather than constructing a new string for every span (simple local refactor)).
• For BreakSpaces, generate additional soft line-breaking opportunities. This is implemented within the line-breaking code rather than in the analysis.
• Implement the different "hanging" behaviour for the various white-space-collapse modes.
• Account for the differences in min/max content widths of the various whitespace-collapsing modes (if whitespace hangs that contributes differently to the content widths than if it doesn't hang)

^ The last change is the cause of the snapshot changes, which look like an improvement to me: in the changed layouts, Parley's computed content width now tightly wrapping it's content rather than including trailing whitespace.

I also have some follow-ups (not included here) that:

• Fix the "trimming" behaviour of the whitespace collapsing. Whitespace was previously being trimmed at the start/end of every inline span, but it should only be trimmed at the start/end of the entire layout/paragraph.
• Make WhiteSpaceCollapse a style rather than a weird out-of-band method on the TreeBuilder

See: https://github.com/nicoburns/parley/tree/with-whitespace-style for a branch that includes those changes