Add styling support for Unicode 18.0's LATIN SMALL LIGATURE LONG S WITH DESCENDER S

[MDLC01]

This PR adds support for styling a new Latin letter that was added in Unicode 18.0. The plain letter is U+1DF95, from Latin Extended-G, and the mathematical italic form is U+1D6A6, from Mathematical Alphanumeric Symbols.

The first commit bumps the MSRV to 1.92 (same as Typst), which is necessary to use u32::strict_add_signed to support the negative delta.

This was tested with the following assertions:

const PLAIN: char = '\u{1DF95}';
const ITALIC: &str = "\u{1D6A6}";
assert_eq!(MathStyle::select(PLAIN, None, false, None), MathStyle::Italic);
assert_eq!(to_style(PLAIN, MathStyle::Italic).to_string(), ITALIC);

[mkorje]

Could you add the new char to the doc comment here please?

[mkorje]

The comments I had here matched the bold header from the Unicode chart, so Ligature long s (U+1D6A6) I'd prefer.

[mkorje]

Not entirely related to this PR, but since you've made the change to allow negative deltas, we should change this too then.