docs: rename impure to compat

compat/doc.go

@@ -1,4 +1,4 @@
-// Package impure is a compatibility layer for Lip Gloss that provides a way to
+// Package compat is a compatibility layer for Lip Gloss that provides a way to
 // deal with the hassle of setting up a writer. It's impure because it uses
 // global variables, is not thread-safe, and only works with the default
 // standard I/O streams.