Consider making highlighting less strict.

[mthadley]

As an example, take a look at the Readme on this repo here: https://github.com/liferay/soy-cheat-sheet

Only some of the code snippets have highlighting, and that's because the grammar is strict. It will only highlight a let command if it also is contained inside a template command. Should we allow these cases? How strict should the grammar be? A plain let would be invalid in a normal Soy file, so it is also helpful to not highlight those as a means of feedback.

We should investigate this and consider the best solution for both use cases.

/cc @zenorocha