Add basic {% doc %} tag parsing support

[jamesmengo]

What are you adding in this PR?

https://github.com/Shopify/develop-advanced-edits/issues/441

Adds support for Liquid doc tags ({% doc %}) in the LiquidHTML parser.
This is the basic implementation which we will iterate on.

What's next? Any followup issues?

• I would like to explore implementing autocomplete for the doc tag to have a working example of full iteration of adding a change
• Next would be the param tag.

What did you learn?

• I learned about how the parser tokenizes our input strings
• I learned about how the grammar produces match results and those are combined with mappings to transform the tokenized inputs into structured data.

Before you deploy

• I included a minor bump changeset <- waiting until we built on top of this to add a changeset
• My feature is backward compatible

[jamesmengo]

• Add basic {% doc %} tag parsing support #621 : 2 dependent PRs (#636 , #638 ) 👈 (View in Graphite)
• main

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