[Bug]: Update emitter to set extensions at Path Item Object

[skywing918]

Describe the bug

Update emitter to set extensions at Path Item Object

Actual Specification

openapi: 3.0.0
info:
  title: (title)
  version: 0.0.0
tags: []
paths:
  /widgets/{id}:
    get:
      operationId: Widgets_get
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: string
      responses:
        '200':
          description: The request has succeeded.
          content:
            application/json:
              schema:
                type: string
components: {}

Expected Specification

https://spec.openapis.org/oas/v3.0.0#path-item-object

openapi: 3.0.0
info:
  title: (title)
  version: 0.0.0
tags: []
paths:
  /widgets/{id}:
    get:
      operationId: Widgets_get
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: string
      responses:
        '200':
          description: The request has succeeded.
          content:
            application/json:
              schema:
                type: string
    x-custom: myvalue
components: {}

Reproduction

1. git clone https://github.com/skywing918/typespec-workaround.git
2. tsp compile extension-pathItem.tsp
3. Open /tsp-output/@typespec/openapi3 to get the result

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.

[timotheeguerin]

Dup of #3925