fix: don't concurrently change output profiles

Multiple lipgloss renderer instances can end up using the same default
termenv.Output. This leads to a race condition when manipulating the
ColorProfile concurrently.

We could mutex protect the standard output in termenv, but it feels like
a weak promise, as the rest of the Output wouldn't (and probably
shouldn't) get protected. Protecting it in lipgloss itself would require
a global lock however.

I can't come up with a proper use-case for this scenario, and therefore
we shift this responsibility to the user.

style_test.go

@@ -58,8 +58,6 @@ func TestStyleRender(t *testing.T) {
 }
 
 func TestStyleCustomRender(t *testing.T) {
-	t.Parallel()
-
 	r := NewRenderer(WithColorProfile(termenv.TrueColor), WithDarkBackground(false))
 	tt := []struct {
 		style    Style