Feature: Add validation in visual editor yaml frontmatter

[vezwork]

Todo

• use mock schema to test validation
• use actual yaml frontmatter schema for validation
• apply validation to yaml content on code block initialization
• apply validation to yaml content on save on edit (and clear old underlines)
• underline validation
• show validation error message in popover when hovering underlined validation

Out of scope

• figure out how to pipe existing LSP validation through to the visual editor. Instead, @cscheid came up with a shortcut to import and call the validator in the visual-editor front-end code-block code. It would probably be ideal to pipe it through (Maybe using custom LSP methods?) so we aren't calling the validator in multiple ways I ended up piping existing diagnostics through!

[vezwork]

I found it difficult to figure out how to validate on-save, so I am validating on every edit right now. This should be tested for larger yaml front-matters in case it introduces latency or other issues. It should be easy to address any issues that come up: a quick fix would be to do something like a debounce on edit validation.

[cscheid]

Looks great!, aside from some formatting issues.

[cscheid]

We also want an entry on apps/vscode/CHANGELOG.md.