related to #18147

related to #18147

This option enables extension authors providing PHP support to prevent needless duplication in the completions list. We can remove it when we extract this functionality to a separate extension.

- Introduced an internal boolean property 'overridable'
- Set this property to true while registering configuration
- Support inheriting this property from parent configuration node level

- Use '[...identifiers]` as key

- Register the settings.override setting
- Dynamically update the schema to support intellisense
- Change the language key format to langid or langids separated by comma

- Contirbute parsing of config as an option to config watcher
- Introduce Config model to hold configurations , overrides and necessary data for services
- Refactor config services to use config models
- Add an api in config services to get configuration for a language