perf(ext/fetch): Use the WebIDL conversion to DOMString rather than USVString for Response constructor

[lmalheiro]

Fixes performance issue #12090

Added benchmark response_string which produced the following results before and after the change:

• using USVString:

Benchmark #13: /home/luismalheiro/projects/deno_pr/target/release/deno run cli/tests/testdata/response_string_perf.js 
  Time (mean ± σ):     432.9 ms ±  26.8 ms    [User: 308.8 ms, System: 192.8 ms]
  Range (min … max):   409.5 ms … 485.4 ms    10 runs

• using DOMString:

Benchmark #13: /home/luismalheiro/projects/deno_pr/target/release/deno run cli/tests/testdata/response_string_perf.js 
  Time (mean ± σ):     175.8 ms ±  17.6 ms    [User: 168.6 ms, System: 73.9 ms]
  Range (min … max):   147.3 ms … 211.6 ms    18 runs

Notice that the difference will increase non-linearly for large strings.

[CLAassistant]

All committers have signed the CLA.

[lmalheiro]

Withdrawing it to sort out CLA issues and CI failures. Sorry for that.