Add "comment" type which causes row to be ignored by compiler/central

1. What is the general goal of the feature?
To reduce the number of warnings flagged by ODK central in large ODK documents with tens/hundreds of comment lines.

2. What are some example use cases for this feature?
It makes legitimate warnings more prevalent.
It allows lines to be completely removed from the form manually or automatically on upload.

3. What can you contribute to making this feature a reality?
Willing to test any updates. I do not have any significant coding ability.