Add support for gawk `-M` / `--bignum` mode

[bertysentry]

The current gawk compatibility harness records -M as unsupported and skips the corresponding bignum / MPFR-oriented gawk fixtures.

Current coverage gap:

• src/it/java/io/jawk/GawkMaketestsParser.java treats -M as an unsupported flag today.
• src/it/resources/gawk/skips.properties currently skips mpfrbigint, mpfrbigint2, mpfrcase, mpfrcase2, mpfrfield, mpfrnegzero, mpfrnegzero2, mpfrnonum, mpfrnr, mpfrrem, mpfrrndeval, mpfrstrtonum, and mpgforcenum because gawk's -M bignum mode is not implemented by Jawk.
• The vendored gawk manifest uses -M / --bignum and switches to $@-mpfr.ok expected files when bignum mode is enabled.

We should add a real bignum mode to Jawk so those cases can run instead of being skipped.

Acceptance criteria:

• Jawk accepts both -M and --bignum on the CLI.
• Arithmetic, conversion, comparison, and formatting semantics are compatible enough to cover the representative vendored fixtures above.
• The compatibility harness can unskip those mpfr* cases and compare against the correct expected output variant.
• The implementation is covered by focused tests plus the existing gawk compatibility suite.