Add getOpenAPI3 to OpenAPI3 emitter (#2950)

This function is useful when you want to work with the OpenAPI 3
directly, or don't want to write it to disk for some reason.

---------

Co-authored-by: Timothee Guerin <timothee.guerin@outlook.com>

.chronus/changes/openapi3-function-2024-1-23-14-2-48.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/openapi3"
+---
+
+Add `getOpenAPI3` function that takes a TypeSpec program and returns the emitted OpenAPI as an object. Useful for other emitters and tools that want to work with emitted OpenAPI directly without writing it to disk.

.chronus/changes/openapi3-function-2024-1-26-14-26-10.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/versioning"
+---
+
+Export the VersionProjections interface.

packages/openapi3/src/lib.ts

@@ -243,6 +243,6 @@ export const libDef = {
 } as const;
 
 export const $lib = createTypeSpecLibrary(libDef);
-export const { reportDiagnostic, createStateSymbol } = $lib;
+export const { createDiagnostic, reportDiagnostic, createStateSymbol } = $lib;
 
 export type OpenAPILibrary = typeof $lib;