add basic support for declaring schema with inline $schema

[bollwyvl]

What does this PR do?

• adds basic support for $schema
  • this has second priority
    • # yaml-language-server $schema=
    • this feature
    • customSchemaProvider

What issues does this PR fix or reference?

• Use schema from "$schema": for current JSON file #964

Is it tested? How?

• test/fixtures/sample-dollar-schema.json added
• test/schema.test.ts
  • the modeline test is updated to include a $schema (which it ignores)
  • a new test is added which only provides a $schema (which beats settings)

[coveralls]

coverage: 84.248% (+0.07%) from 84.174%
when pulling b29478a on bollwyvl:gh-964-use-dollar-schema
into f039273 on redhat-developer:main.

[bollwyvl]

Looking more through older PRs and issues:

• enable schema self identification using inline $schema #437
• support specifying schema via $schema key #354

it appears some of the relevant argument there was in somehow ignoring $schema in additionalProperties... this PR doesn't propose anything like that, and the # yls modeline or by configuration would indeed be the only way in such a case.

[bollwyvl]

I see 1.15.0 went out 🎉... what can I do to help this get into the review pipeline, and hopefully make it into the next release?