PNE TOC enabled filter on type for markdown and code cells

## Context

Followup ticket to https://github.com/posit-dev/positron/issues/9638 

## AC
1. Include filter
<img width="536" height="106" alt="Image" src="https://github.com/user-attachments/assets/1033b48a-c93e-468a-b26a-0293b8c6d9f0" />

2. Allow for filtering on
a. Markdown cells only
b. code cells only 

3. Should work with both Python and R code