* 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

Fixes #47760

Fixes #48709

Use a unified diagnostic collection for js and ts since both diagnostics are provided by the typescript extension

These diagnostics should only ever be downgraded from errors to warnings, not ever upgraded from a lower level to warnings

* Support basic suggest diagnostics from TS

* Removing duplicate diagnostic paths

* Avoid duplicate code around handling multiple diagnostic kinds

String paths should only be used when talking to the ts server

Better decouple host from client

Better decouple the language host from the language clients by using events instead of direct calls

String paths should only be used when talking to the ts server

Better decouple host from client

Better decouple the language host from the language clients by using events instead of direct calls

Fixes #41116

Fixes #28214

Part of #39329