Format rest of specs

In https://github.com/WICG/webmonetization/commit/cbb2bee6d98750d959fc068b5b7fb10b7e2d42dc I added a format command for the Web Monetization spec. This does not yet include the Extension API spec, or the Flows spec.

These specs use includes, which are a bit tricky with tidy-html. In the end we want all index.html and includes to be formatted accordingly. The only way to somewhat format the includes was to add the following tidyconfig options:

omit-optional-tags: yes
doctype: omit

With this, it does not add a full HTML skeleton to the includes, however, that has issues that will need to be solved:

• We need to decide if omitting optional end tags is something we are okay with (we for example use quite a lot of p tags)
• tidyhtml adds an empty <title> to each include, which we don't want
• There is a bug in the media-example.html include if we apply formatting. For audio and video tags the link tag is a child node. For picture it gets moved out.

<picture srcset="cat.jpeg" onmonetization="sayThanks(this)"></picture>
<link rel="monetization" href="https://example.com/images/pay">

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WIP branch: https://github.com/MichaelKohler/webmonetization/tree/wip-format-specs

Start by reviewing the format command from commit cbb2bee6d98750d959fc068b5b7fb10b7e2d42dc and the WIP branch, then inspect the Extension API and Flows specs, their index.html files, and includes. Check the tidyconfig options and media-example.html behavior before deciding how optional tags and empty titles should be handled. Done means all index.html files and includes are formatted without the incorrect picture/link structure.