Report diagnostics for unused "using" so that they will be shown properly in ide

[RodgeFu]

When "using" and "import" are not needed, dim them and provide quickfix in IDE. (Extracted from the PR #5146 for the unused using part)

Fixes: #3373

[azure-sdk]

All changed packages have been documented.

• ✅ @typespec/compiler
• ✅ @typespec/http-server-csharp
• ✅ @typespec/http-specs
• ✅ @typespec/http
• ✅ @typespec/openapi
• ✅ @typespec/openapi3
• ✅ @typespec/rest
• ✅ @typespec/streams
• ✅ @typespec/xml

Show changes

@typespec/compiler - feature ✏️

Support diagnostics for unused using statement

@typespec/http-specs - internal ✏️

Fix test for diagnostics of unused using statement

@typespec/http - internal ✏️

Fix test for diagnostics of unused using statement

@typespec/openapi - internal ✏️

Fix test for diagnostics of unused using statement

@typespec/openapi3 - internal ✏️

Fix test for diagnostics of unused using statement

@typespec/rest - internal ✏️

Fix test for diagnostics of unused using statement

@typespec/streams - internal ✏️

Fix test for diagnostics of unused using statement

@typespec/xml - internal ✏️

Fix test for diagnostics of unused using statement

@typespec/http-server-csharp - internal ✏️

Fix test for diagnostics of unused using statement

[azure-sdk]

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