Resolves #4258 — the digit-separator work in #4280 covered decimal and hex literals, but a few C# number forms are still highlighted wrong.

Changes

The number mode had kept some C/C++ habits: ' as the digit separator, _ allowed only in the decimal/hex variant, and the u/U/l/L suffixes recognised only after a plain decimal literal. In practice that means:

int bin = 0b1010_1010;      // stops highlighting at the first _
ulong hex = 0xFF_FFUL;      // UL falls out of the literal
ulong dec = 1_000_000UL;    // same
decimal price = 19.99m;     // m/M and d/D suffixes aren't matched at all
int _count = 0;             // the leading _ renders as a number

The last one is the one I noticed first — [\d_]+ can start with an underscore, so _count (the usual private-field convention) gets a stray highlighted _, and point._1 highlights ._1.

I rewrote the three variants to follow the lexical grammar in the spec (integer literals, real literals): separators sit between digits and may also follow the 0x/0b prefix, integer literals take U/L in either order, and real literals take F/D/M.

Tested with a new markup fixture covering the forms above plus the two identifier cases; npm run build && npm test is green (1579 passing), as are npm run lint and npm run lint-languages. The existing csharp/floats fixture is unchanged.

Checklist

• Added markup tests, or they don't apply here because...
• Updated the changelog at CHANGES.md