perf: run two JSON API fetches in parallel

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes ⚡️ Performance: Parallelize API calls when possible #149
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Uses Promise.all and a new fetchAsJson helper to clean up the code a bit and run the two requests in parallel, rather than in series.

[codecov]

Codecov Report

Attention: 27 lines in your changes are missing coverage. Please review.

Comparison is base (2cb7789) 10.50% compared to head (ef00b26) 10.93%.

❗ Current head ef00b26 differs from pull request most recent head 660b265. Consider uploading reports for the commit 660b265 to get more accurate results

Files	Patch %	Lines
src/content-script.ts	0.00%	23 Missing ⚠️
src/fetchAsJson.ts	0.00%	3 Missing ⚠️
src/types.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #170      +/-   ##
==========================================
+ Coverage   10.50%   10.93%   +0.42%     
==========================================
  Files           5        6       +1     
  Lines         314      320       +6     
  Branches        8        8              
==========================================
+ Hits           33       35       +2     
- Misses        281      285       +4     

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