Adds new check for usage of valid types

[MateusJabour]

What are you adding in this PR?

Adds a new check to make sure that a static block is using a valid block type, one that exists in the directory of blocks.

Read more https://docs.google.com/document/d/1d1EpwfhbgSubLjEqMePZ30ZWJovuCt2mJQslgWSwex0/edit#heading=h.8i0fnctjpka9

Before you deploy

• [x] This PR includes a new checks or changes the configuration of a check
  • [ ] I included a minor bump changeset
  • [ ] It's in the allChecks array in src/checks/index.ts
  • [ ] I ran yarn build and committed the updated configuration files
  • [ ] If applicable, I've updated the theme-app-extension.yml config

• [ ] I included a minor bump changeset
• [ ] My feature is backward compatible

• [ ] I included a patch bump changeset