What does this PR do?

To aid interop with JetBrains' IntelliJ based IDEs, this adds support for specifying an inline-schema through # $schema: <url>. This is now the preferred way to specify an inline-schema. For backwards compatibility, the old way is still supported.

What issues does this PR fix or reference?

Fixes #959.

Is it tested? How?

Tested it with the yaml-vscode extension (and in unit tests).

1. Below the comment, list the available completion items (in VS Code: CTRL+Space)

   # $schema: https://json.schemastore.org/clangd.json

2. Same here, list the available completions below the comment

   # yaml-language-server: $schema=https://json.schemastore.org/clangd.json

3. • In an empty file, list the available completions.
   • It should show Inline Schema. Use it.
   • It should complete to # $schema: with the cursor at the end.
   • List the available completions. It should show all available schemas.