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 ?