* Add unused diagnostic subtype

Fixes #15710

Adds a new `DiagnosticTag` class that provide additional information about a diagnostic. Introduce `DiagnosticTag.Unnecessary` to mark when a diagnostic is for unused / unnecessary code

The design comes from Rosyln's diagnostic object and allows us to modify how a diagnostic is rendered without changing its serverity.

Hooks up JS and TS to use this new tag. This is controlled by the `javascript.showUnused.enabled` setting which is enabled by default

- Introduce a new diagnostic severity for unused.

    However, using this approach, if a user sets `noUnusedLocals` in their `tsconfig.json`, the resulting diagnostic could only show the squiggly OR be grayed out. Using `customTags` allows us to support both graying out and showing the squiggly

- Custom JS/TS implementation using decorators

    Not themable. We want a standard experience across languages.

* - Move to proposed
- Use numeric enum