feat(lexarg): Decouple compatibility surface, allowing for helpers

[epage]

By pulling out lexarg_parser, lexarg can have more helpers and more general policy. This also allowed reducing the API surface of lexarg_error.

This does come at the cost of more crates to compile and we're forcing more code on people but hopefully the compile time costs are not too great.

[coveralls]

Pull Request Test Coverage Report for Build 14113534492

Details

• 128 of 284 (45.07%) changed or added relevant lines in 5 files are covered.
• 13 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.9%) to 28.397%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/lexarg-error/src/lib.rs	0	10	0.0%
crates/libtest2-harness/src/harness.rs	0	14	0.0%
crates/lexarg-parser/src/lib.rs	128	157	81.53%
crates/lexarg/src/lib.rs	0	47	0.0%
crates/libtest-lexarg/src/lib.rs	0	56	0.0%

Files with Coverage Reduction	New Missed Lines	%
crates/lexarg-error/src/lib.rs	3	0.0%
crates/libtest-lexarg/src/lib.rs	4	0.0%
crates/libtest2-harness/src/harness.rs	6	0.0%

Totals
Change from base Build 14110905723:	-0.9%
Covered Lines:	326
Relevant Lines:	1148

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💛 - Coveralls