Use AST parser to validate alt text - markdown-lint

[kendallgassner]

What

Closes #30

I updated the alt-text-bot to use an AST (markdown-lint) to parse images for bad alt text. Using an AST allows us to ignore any images defined in code blocks.

Why is this cool

• In the future, we can add additional linting rules
• The workflow is easier to read in javascript
• We can test more easily
• We can ensure that there aren't false positives
• We can provide more feedback. For example, it is easier to indicate what line the alt text is found on.

[kendallgassner]

Proof of concept run: https://github.com/khiga8/link-checker/issues/9

@khiga8 ... i had to check in node_modules..... but it works

[khiga8]

Eeep sorry I didn't get to review this today, I'll take a look first thing tomorrow!

[kendallgassner]

Checking in node_modules as advised on github action documentation.