Add typekit to list types under a container (namespace/interface)

[Copilot]

This PR adds a new utility function to the typekit that allows listing all types under a container (namespace or interface) that match a specific filter criteria.

The new function is implemented as:

$.type.listUnder(container, (type) => filter)

This utility provides a consistent way to list types under a namespace or interface and apply a filter to find types with specific characteristics, which is a common need in libraries consuming TypeSpec types.

Example usage

// List all models in a namespace
const models = typespecType.listUnder(
  namespace, 
  (type): type is Model => type.kind === "Model"
);

// List all types with a specific decorator
const withMyDec = typespecType.listUnder(
  namespace, 
  (type) => type.decorators.some(d => d.decorator.name === "@myDec")
);

// List operations from a specific namespace with non-recursive search
const operations = typespecType.listUnder(
  namespace,
  (type) => type.kind === "Operation",
  { recursive: false }
);

The implementation is based on the pattern used by listOperationsIn helper, but generalized to work with any type category.

Fixes #7367.

Warning

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[timotheeguerin]

this file is also in the wrong location

[Copilot]

Moved the type-utils.test.ts file to the correct location in the test directory and updated imports in 9a633f8.

[timotheeguerin]

this should be added to packages/compiler/test/typekit/type.test.ts

[Copilot]

I've moved the test file to packages/compiler/test/typekit/type.test.ts in b4d3801.