Add optional path parameters

[CathalMullan]

Realising that with this approach, we can have any number of optionals, anywhere in a route.

Not sure if that's a good thing or not.

Long term, I'd like to add a real parser that can do this in one pass, but the additive approach is fine for now. Once we have benchmarks for inserts/deletes, that will help. Syntax isn't really "set" yet, either.

This is 'good enough' for now.

[codecov]

Codecov Report

Attention: Patch coverage is 84.03990% with 64 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/expander.rs	80.82%	0 Missing and 37 partials ⚠️
src/parser.rs	90.76%	5 Missing and 7 partials ⚠️
src/router.rs	76.59%	10 Missing and 1 partial ⚠️
src/node/delete.rs	75.00%	4 Missing ⚠️

Files with missing lines	Coverage Δ
src/lib.rs	60.46% <ø> (+4.90%)	⬆️
src/node/insert.rs	93.26% <100.00%> (+1.50%)	⬆️
src/node/search.rs	79.20% <100.00%> (+0.48%)	⬆️
src/node/delete.rs	77.77% <75.00%> (-0.61%)	⬇️
src/router.rs	84.57% <76.59%> (-3.11%)	⬇️
src/parser.rs	96.70% <90.76%> (-1.90%)	⬇️
src/expander.rs	80.82% <80.82%> (ø)

[codspeed-hq]

CodSpeed Performance Report

Merging #147 will improve performances by 5.37%

_{Comparing 95-support-optional-path-parameters (75efabb) with main (148353a)}

Summary

⚡ 1 improvements
✅ 15 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	95-support-optional-path-parameters	Change
⚡	matchit benchmarks/path-tree	13.9 µs	13.2 µs	+5.37%