feat(compiler): Add interface typekit ($.interface)

[Copilot]

Interfaces were the only major TypeSpec type without a corresponding typekit entry. This adds $.interface with create and is methods, consistent with existing kits for model, operation, enum, etc.

Changes

• kits/interface.ts — New InterfaceKit with:
  • create(desc: InterfaceDescriptor): Interface — builds a finished Interface type, optionally with pre-built operations
  • is(type: Entity): type is Interface — type guard
• kits/index.ts / typekit/index.ts — Export InterfaceKit and InterfaceDescriptor
• test/typekit/interface.test.ts — Tests for is, create (empty), and create with operations

Usage

const op = $(program).operation.create({
  name: "list",
  parameters: [],
  returnType: $(program).intrinsic.void,
});

const iface = $(program).interface.create({
  name: "PetService",
  operations: [op],
});

$(program).interface.is(iface); // true
iface.operations.get("list");   // op, with op.interface === iface

Original prompt

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This section details on the original issue you should resolve

<issue_title>[compiler] There is no typekit for interfaces</issue_title>
<issue_description>### Clear and concise description of the problem

This seems like an oversight. We have typekits for everything else, but none for making an interface.

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.</issue_description>

<agent_instructions>Follow the examples of other typekits in packages/compiler/src/typekit/kits to create a versatile typekit for creating instances of interfaces.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [compiler] There is no typekit for interfaces #9770

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