Feature request: Case insensitive enum values

It'd be nice to have a way to mark a enum to be case insensitive, i.e. to do case insensitive comparison of the actual value in the document with the items of the enumeration during validation.

There was some discussion around this use case in JSON schema mailing list quite a while ago, and in general there're legit concerns about feasibility of anything like this in the JSON schema itself. However I believe it makes sense as a custom validator extension. As an example, IntelliJ introduced x-intellij-case-insensitive extension parameter for this use case recently WEB-39440.

Start by locating the language server's enum-validation entry point and any existing extension-handling code; the issue names no files or tests. Define how the custom parameter should be expressed and how matching should behave, then add coverage for case-insensitive enum validation before considering the work done.