Fix StackOverflow when selecting a schema for a YAML file that matches multiple schemas

[shin19991207]

What does this PR do?

yaml-language-server supports validating a YAML file against multiple matching JSON schemas by creating an internal combined schema with JSON Schema allOf. Because multiple schemas can be valid for one YAML file, the schema picker should allow users to select or deselect multiple schemas instead of forcing a single choice.

This PR updates the schema picker behavior:

• supports selecting/deselecting multiple schemas from the schema picker
• treats "No JSON Schema" option as an override: if selected, other schema selections are ignored and schema detection is disabled for the current YAML file.
• status bar is updated when a modeline schema is added or changed in the current YAML file
• changed version selection to a per-schema action: to select a schema version, hover over a schema name in the picker, click the versions icon on the right to choose/see available versions:
  

What issues does this PR fix or reference?

Fixes #1249
Fixes #983
Related to redhat-developer/yaml-language-server#1260

Is it tested? How?

• New automated tests
• Manual testing: tested with Fix handling of absolute local schema paths from yaml.schemas and yaml/get/jsonSchema StackOverflow yaml-language-server#1260

[datho7561]

I was able to select "No Schema" in addition to two schemas. I think it's not evident what this does to the end user. Maybe it's worth deselecting the other schemas when you select "No Schema"

[datho7561]

Looks pretty good from my testing, outside of #1253 (comment) which I think will be confusing for users and could be improved.

[shin19991207]

@datho7561 Thanks for the review and the suggestion! I updated the behavior now:

• selecting "No JSON Schema" clears all other selected schemas immediately
• selecting a schema while "No JSON Schema" is selected clears "No JSON Schema" immediately
• if everything is deselected, the picker auto-selects "No JSON Schema"

[datho7561]

Looks good. Thanks, Morgan!