Precompile snippets

[Witiko]

As mentioned by @josephwright in latex3/latex3#1542 and in their TUG talk, the speed of repeatedly setting the same key–values can be improved by precompiling the key–values (also referred to as "instantiation" in the context of LaTeX templates).

In the Markdown package, snippets are essentially key–values for the \markdownSetup command that we store for later use. Precompiling (frequently used) snippets should enhance speed without affecting correctness.