Warn on deploy if set_language isn't at the top of a schema.

[kdutia]

Is your feature request related to a problem? Please describe.
I've implemented set_language to set the two documents in my schema to English. A PR review picked up the note in your docs that the field should be the first field in the document. I've changed this, however am worried that future developers on our schema might not know this and might put a field above it which is then unaffected by this language setting.

Describe the solution you'd like
I find warnings about schemas really useful when doing a local deploy, and thought this would be a useful extra warning to add.

Describe alternatives you've considered
Adding a comment to the schema for future developers instead, but I can't figure out whether it's possible to add comments to .sd files.

Additional context
Add any other context or screenshots about the feature request here.