Add support for specifying schema via # $schema: …

[pascal-hofmann]

Is your enhancement related to a problem? Please describe.

JetBrains recently added support for specifying YAML schema via inline comment to their products. This looks similar to the modeline that can be used for yaml-language-server.

# $schema: …

Examples:

# $schema: https://example.com/path/to/schema.json
# $schema: ./schema.json

Describe the solution you would like

It would be great if yaml-language-server would support both ways (existing modeline, plus $schema: …) of specifying the schema inline, so we don't have to specify it twice.

Describe alternatives you have considered

None.

Additional context