Add math extension functions

[rayokota]

What

Adds a math extension exposing CEL's namespaced numeric functions, enabled the
same way as the existing strings extension:

import { math } from "@bufbuild/cel/ext";                                                                                                                                         
                                                                                                                                                                                  
run(`math.abs(-4) == 4`, {}, { funcs: math }); // true     

Functions: abs, sign, sqrt, ceil, floor, round, trunc, isInf,
isNaN, isFinite, bitAnd, bitOr, bitXor, bitNot, bitShiftLeft,
bitShiftRight. Ported from cel-go's ext/math.go

Not included: math.greatest / math.least

These are variadic receiver macros, which require parser-macro support that
extensions don't have today. They're intentionally left out and their conformance
cases remain skipped. I'm happy to follow up with the macro mechanism if there's
interest in a design discussion.

Notes

• Additive only — no changes to any exported API shape. math is a CelFunc[], matching strings.
• Semantics match cel-go: round is half-away-from-zero, right shift is logical,
  abs(minInt64) errors, shifts ≥ 64 yield 0, negative shifts error.
• The conformance suite's non-macro math_ext cases are un-skipped (only
  greatest/least remain skipped).

Testing

• Full suite green; the newly enabled math_ext conformance cases pass.
• attw clean for @bufbuild/cel and @bufbuild/cel/ext.
• README updated with a math example.

[CLAassistant]

All committers have signed the CLA.