feat: make Lipgloss Style thread-safe (mutex)

[muesli]

This is related to #139, but uses a sync.RWMutex based approach to fully guard the Style struct.

Since we had a few reports about concurrently accessing base style definitions, I figured one solution would be making Style's rules map thread-safe. While it's an elegant solution, it is a trade-off: it guards against crashes from concurrent access, but that obviously slows down access by a tiny bit. I don't think the impact is too bad, though, and not crashing is always nice.

go test -bench=. -benchtime=30s

Before change:

goos: linux
goarch: amd64
pkg: github.com/charmbracelet/lipgloss
cpu: Intel(R) Core(TM) i5-6200U CPU @ 2.30GHz
BenchmarkStyleRender-4           7875594              4555 ns/op
PASS

With sync.RWMutex:

goos: linux
goarch: amd64
pkg: github.com/charmbracelet/lipgloss
cpu: Intel(R) Core(TM) i5-6200U CPU @ 2.30GHz
BenchmarkStyleRender-4           5798413              6138 ns/op
PASS