Support editing JSON in ScriptEditor

[Paulb23]

Added support to edit JSON in the ScriptEditor with some base functionality.

Some parts of the ScriptEditor have been generalised to support this.

JSON file types are opened with the TextEditor class used for TextFiles, rather then duplicating this again (until this is refactored). The TextEditor will will report and highlight errors reported by the JSON parser.

Finally, added a basic JSONSyntaxHighlighter. The ScriptEditor previously used to only look at the script language, now if it is not a script it will look at the extension. This is to allow using the JSON highlighter for JSON files and PlainTextHighlighter for TextFiles.

supersedes #67739
closes #66820

[akien-mga]

Needs rebase to pass CI, I fixed the GDScript test error.

[akien-mga]

Thanks!