[openapi3] Add missing peer dependency "openapi-types" (microsoft#5274)

Fixes warnings (or potentially errors) when installing
`@typespec/openapi3`:

```
$ npx yarn add @typespec/[email protected]

yarn add v1.22.22
info No lockfile found.
[1/4] Resolving packages...
[2/4] Fetching packages...
[3/4] Linking dependencies...

warning " > @typespec/[email protected]" has unmet peer dependency "@typespec/compiler@~0.62.0".
warning " > @typespec/[email protected]" has unmet peer dependency "@typespec/http@~0.62.0".
warning " > @typespec/[email protected]" has unmet peer dependency "@typespec/versioning@~0.62.0".
warning " > @typespec/[email protected]" has unmet peer dependency "@typespec/openapi@~0.62.0".

warning "@typespec/openapi3 > @readme/[email protected]" has unmet peer dependency "openapi-types@>=7".
```

The first 4 warnings are expected, but the peer dependency on
`openapi-types` should either be satisfied by `@typespec/openapi3`
itself, or be declared as a peer dependency of `@typespec/openapi3`
itself.

Author: mikeharder

.chronus/changes/openapi3-peer-2024-11-5-18-44-45.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/openapi3"
+---
+
+Added missing peer dependency "openapi-types"

packages/openapi3/package.json

@@ -59,6 +59,7 @@
   ],
   "dependencies": {
     "@readme/openapi-parser": "~2.6.0",
+    "openapi-types": "~12.1.3",
     "yaml": "~2.5.1"
   },
   "peerDependencies": {