Add HTML multipart behavior compatibility

[blakeembrey]

Builds upon the demo parser/server PR.

Adds multipart behavior support based on the HTML spec. Will be useful for handling browser uploads. In format, it does not encode as filename* (matching browser behavior) but on server it does decode filename* since it would need to match more than browser behavior, such as better formed uploads.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (54302a9) to head (bea27c4).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #131   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          227       281   +54     
  Branches        82       102   +20     
=========================================
+ Hits           227       281   +54     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.