fix: don't warn on .text() usage with image/svg+xml

[Cherry]

After bumping some dependencies in a project, I started seeing this log in my tests:

workerd/io/worker.c++:1138: info: console warning; message = Called .text() on an HTTP body which does not appear to be text. The body's Content-Type is "image/svg+xml; charset=utf-8". The result will probably be corrupted. Consider checking the Content-Type header before interpreting entities as text.

This PR updates the detection here to also allow image/svg+xml. As per https://datatracker.ietf.org/doc/html/rfc6838, it seems that allowing +xml at the end of any mimetype is fine to be treated as text:

This document also standardizes a convention (using the suffix '+xml') for naming media types ... when those media type represent XML MIME (Multipurpose Internet Mail Extensions) entities.

Let me know if any tweaks are needed - I included tests where it made sense.

[Cherry]

That benchmark seems unrelated and spikes every so often in unrelated changes as per https://codspeed.io/cloudflare/workerd/benchmarks/src/workerd/tests/bench-response.c%2B%2B::bodyWithHeaders%5BResponse%5D?runnerMode=Simulation&branchId=69ab239b9c8e9085e4f270c0.

If that's not the case, please let me know.

[anonrig]

Yes, benchmark is flaky. No need to worry about that. I've rerun the CI.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 11.11111% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.67%. Comparing base (fd0665d) to head (9e53aeb).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/workerd/util/mimetype-test.c++	6.25%	0 Missing and 15 partials ⚠️
src/workerd/util/mimetype.h	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6269      +/-   ##
==========================================
- Coverage   70.69%   70.67%   -0.03%     
==========================================
  Files         417      417              
  Lines      111111   111127      +16     
  Branches    18331    18346      +15     
==========================================
- Hits        78552    78536      -16     
- Misses      21614    21630      +16     
- Partials    10945    10961      +16     

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