Place the hidden svg/defs elements at the beginning of the document, not the end.

[dpvc]

This PR changes where the MathDocument puts additional elements into the page. Currently, only the SVG output has any of these, and they are the svg element used to store the global font cache (in a defs element). This should be at the top of the document, not the bottom, so that the definitions are in place before the elements that use them (important when the document is preprocessed on the server).

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 86.71%. Comparing base (4d1e195) to head (82db2ae).
Report is 38 commits behind head on develop.

Files with missing lines	Patch %	Lines
ts/handlers/html/HTMLDocument.ts	0.00%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1308      +/-   ##
===========================================
- Coverage    86.71%   86.71%   -0.01%     
===========================================
  Files          337      337              
  Lines        83979    83985       +6     
  Branches      4750     4750              
===========================================
  Hits         72826    72826              
- Misses       11130    11136       +6     
  Partials        23       23              

☔ 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.

[zorkow]

lgtm.