Fix incorrect generic type parameter (DefinitelyTyped#58082)

The introduction of the `Document` type [in this PR](postcss/postcss#1582) incorrectly advertises that `parse` can return a `Document` object, because the [`Parser` interface is now generic](https://github.com/postcss/postcss/pull/1582/files#diff-68ba6abc949516587990ca61016794e81039fd6ae58437d5cfaccd4eeebdac07R224).

This PR exposes the parser as a more specific type of `Parser` so that consumers aren't given a `Root | Document`, but just a `Root`.

types/postcss-less/index.d.ts

@@ -10,7 +10,7 @@ import * as postcss from 'postcss';
 export = postcssLess;
 
 declare const postcssLess: postcss.Syntax & {
-    parse: postcss.Parser;
+    parse: postcss.Parser<postcss.Root>;
     stringify: postcss.Stringifier;
     nodeToString: (node: postcss.Node) => string;
 };