Add KDL format support (v1 + v2)

[TomWright]

Summary

• Add KDL (KDL Document Language) as a new input/output format (-i kdl / -o kdl), supporting both v1 and v2 syntax with automatic version detection
• Hand-written tokenizer, parser, and generator with zero external dependencies
• Maps KDL's node-oriented model to dasel's ordered maps: scalar nodes become direct values, duplicate names promote to slices, mixed nodes use $args for arguments and properties as map keys
• Configurable output version via --write-flag kdl-version=1 (bare true/false/null) or --write-flag kdl-version=2 (#true/#false/#null). Defaults to v2

Closes #504

Test plan

• Internal tokenizer tests (v1 + v2 syntax, escapes, raw strings, multi-line strings, comments, numbers)
• Internal parser tests (all node forms, slashdash, type annotations, version markers)
• Internal generator tests (round-trip, compact mode, escaping, v1 vs v2 output)
• Official KDL v2 spec test suite (73 tests from kdl-org/kdl test cases)
• Official KDL v1 spec test suite (28 tests)
• v1↔v2 conversion tests (20 tests verifying identical ASTs and round-trips)
• Reader/writer integration tests (22 tests including version flag)
• Cross-format tests: KDL↔JSON, KDL↔YAML, JSON→KDL, YAML→KDL (28 tests)
• CLI-level cross-format tests in generic_test.go (33 subtests)
• Full test suite passes with zero regressions