🏗🚀 Speed up validated example grouping ~45x (#34837)
🏗🚀 Speed up validated example grouping ~45x (#34837)

Speeds up the format grouping process of `amp validate-html-fixtures` by about 45x.

This represents a `~24%` speedup of the overall task.

- Uses `posthtml` instead of `JSDOM` to parse the tree when necessary. 
- Filters out files that lack a suffix like `4email`.

### Performance comparison

| method   | filters | time (ms) |          |
| -------- | ------- | --------: | -------- |
| JSDOM    |         |      2095 | (before) |
| JSDOM    | yes     |       241 |          |
| posthtml |         |       116 |          |
| posthtml | yes     |        47 | (after)  |

jridgewell/amphtml,alanorozco/amphtml,rsimha-amp/amphtml,media-net/amphtml,Gregable/amphtml,danielrozenberg/amphtml,ampproject/amphtml,jridgewell/amphtml,jpettitt/amphtml,rsimha-amp/amphtml,honeybadgerdontcare/amphtml,ampproject/amphtml,mhc-gh/amphtml,danielrozenberg/amphtml,jpettitt/amphtml,media-net/amphtml,jpettitt/amphtml,Gregable/amphtml,danielrozenberg/amphtml,alanorozco/amphtml,honeybadgerdontcare/amphtml,honeybadgerdontcare/amphtml,rsimha-amp/amphtml,alanorozco/amphtml,ampproject/amphtml,Gregable/amphtml,jridgewell/amphtml,jridgewell/amphtml,mhc-gh/amphtml,ampproject/amphtml,smartadserver/amphtml,ampproject/amphtml,jridgewell/amphtml,danielrozenberg/amphtml,smartadserver/amphtml,danielrozenberg/amphtml,Gregable/amphtml,jpettitt/amphtml,Gregable/amphtml,rsimha-amp/amphtml,media-net/amphtml,Gregable/amphtml,media-net/amphtml,media-net/amphtml,smartadserver/amphtml,ampproject/amphtml,alanorozco/amphtml,alanorozco/amphtml,ampproject/amphtml,honeybadgerdontcare/amphtml,honeybadgerdontcare/amphtml,mhc-gh/amphtml,Gregable/amphtml,honeybadgerdontcare/amphtml,rsimha-amp/amphtml,ampproject/amphtml,smartadserver/amphtml,media-net/amphtml,jpettitt/amphtml,rsimha-amp/amphtml,danielrozenberg/amphtml,danielrozenberg/amphtml,jpettitt/amphtml,alanorozco/amphtml,smartadserver/amphtml,jridgewell/amphtml,rsimha-amp/amphtml,smartadserver/amphtml,mhc-gh/amphtml,mhc-gh/amphtml,alanorozco/amphtml,Gregable/amphtml,media-net/amphtml,mhc-gh/amphtml,jridgewell/amphtml,media-net/amphtml,alanorozco/amphtml,jpettitt/amphtml,mhc-gh/amphtml,mhc-gh/amphtml,danielrozenberg/amphtml,honeybadgerdontcare/amphtml,jpettitt/amphtml