Add TreeBuilder push/build benchmarks

[nicoburns]

LLM Contributions: Code and this description were written by Devin (an LLM agent). This PR is kept for reference and is not expected to be merged.

This PR is stacked on #62. It adds TreeBuilder push and TreeBuilder build benchmarks to parley_bench, which were used to measure the effect of removing uncommitted_text in #62.

• TreeBuilder push pushes a sample text through push_style_span, push_text and pop_style_span and does no shaping. It varies the number of chars per push_text call (1 or 10), the number of pushes per span (1, 8 or 10) and the WhiteSpaceCollapse mode.
• TreeBuilder build does the same pushes, followed by build_into, which includes shaping.

Tango results against the buffered builder are in #62.

Changelog: None

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/4641a2bf5eca4d4fb140d16886180423
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/4641a2bf5eca4d4fb140d16886180423?variant=devin-insiders
Requested by: @nicoburns

[staging-devin-ai-integration]

I'll fix CI failures and address comments from users with write access that start with 'Devin'.

• Disable automatic comment, CI, and merge conflict monitoring