canonicalize double exclamation !! to speak as double factorial

[AdamMagued]

Resolves #806.

When !! is encountered in <mo>!!</mo>, it was previously left as literal ASCII exclamation marks and spoken as "5 !!" rather than "5 double factorial".

This change:

1. Canonicalizes "!!" to "\u{203C}" ("‼") in canonicalize_mo_text, aligning with the existing rules for double factorial across languages.
2. Adds "‼" to operator-info.in with postfix operator type and priority 810 (matching single and double factorial operators).
3. Adds a test in symbols_and_adornments.rs verifying that <math><mn>5</mn><mo>!!</mo></math> produces "5 double factorial" in ClearSpeak and SimpleSpeak.

[github-actions]

Linux library size: 0.58 MiB (0.00%)

Revision	Release liblibmathcat.so
Base (3bbbd44)	0.58 MiB (605,312 bytes)
PR (846327e)	0.58 MiB (605,312 bytes)
Change	0 bytes (0.00%)

Built with default features, Rust 1.96.0, and Ubuntu 24.04. Workflow run.

[moritz-gross]

When I wrote this issue, I didn't even catch that it's already covered and just needs to be normalized.
PR looks good to me.

@NSoiffer what do you think? Also, I've not really worked with the operator-info.in table yet, but isn't there an easier/safer way to get this info? As I understand it, this is based on the W3C standard, which at this point even changed the priority in MathML4 to 820 instead of 810 (search for !, !!, %, ′, it's located in appendix B 3.1). To me, the ideal scenario would be a 3rd party knowledge base to use, just like we did with elements-rs for chemistry.
Couldn't we just parse https://github.com/w3c/xml-entities/blob/gh-pages/unicode.xml ?

[AdamMagued]

Hi @moritz-gross,

Thank you for the review and the insight! Normalizing it so !! canonicalizes consistently to double factorial ensures speech synthesis and navigation behave as expected across different MathML representations. Happy to make any further adjustments if @NSoiffer or the team have suggestions!

[moritz-gross]

@AdamMagued Neil is busy for a few days now, so we likely won't get to review this PR in this week. We'll comment if anything comes up. thanks for your work