LibGUI: Support multiple layers of TextDocument spans

TextDocument::set_spans() now also takes a "span collection index"
argument.

TextDocument keeps a map between a span collection index and its spans.
It merges the spans from all collections into a single set of spans
whenever set_spans() is called.

This allows us to style a document with multiple layers of spans, where
as previously we only supported a single layer of spans that was set
from the SyntaxHighlighter.

Userland/Libraries/LibGUI/TextEditor.cpp

@@ -1951,7 +1951,7 @@ void TextEditor::set_syntax_highlighter(OwnPtr<Syntax::Highlighter> highlighter)
         m_highlighter->attach(*this);
         m_needs_rehighlight = true;
     } else
-        document().set_spans({});
+        document().set_spans(Syntax::HighlighterClient::span_collection_index, {});
     if (on_highlighter_change)
         on_highlighter_change();
 }