Enable 'eslint-plugin-regexp' and fix existing findings

[calixteman]

Enable the recommended preset and fix or per-line-disable the 78 findings it surfaces. Most are equivalent rewrites, intentional patterns (control chars, the whatwg email regex, autolinker URL regex) keep their behavior via targeted disables.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.42%. Comparing base (5f2691e) to head (7bda0fc).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
src/display/annotation_layer.js	0.00%	1 Missing ⚠️
src/display/content_disposition.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21330   +/-   ##
=======================================
  Coverage   81.42%   81.42%           
=======================================
  Files         257      257           
  Lines       65339    65352   +13     
=======================================
+ Hits        53202    53216   +14     
+ Misses      12137    12136    -1     

Flag	Coverage Δ
fonttest	9.10% <14.28%> (-0.01%)	⬇️
integrationtest	67.11% <81.81%> (+0.11%)	⬆️
unittest	56.88% <88.00%> (+<0.01%)	⬆️
unittestcli	56.07% <80.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

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[timvandermeij]

r=me, with passing browser tests. Thanks!

[timvandermeij]

/botio browsertest

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Received

Command cmd_browsertest from @timvandermeij received. Current queue size: 1

Live output at: http://54.241.84.105:8877/ea5d365cd68fe59/output.txt

[moz-tools-bot]

From: Bot.io (Windows)

---

Received

Command cmd_browsertest from @timvandermeij received. Current queue size: 1

Live output at: http://54.193.163.58:8877/f52358de0e3c929/output.txt

[moz-tools-bot]

From: Bot.io (Windows)

---

Failed

Full output at http://54.193.163.58:8877/f52358de0e3c929/output.txt

Total script time: 9.74 mins

• Regression tests: FAILED

  errors: 5

Image differences available at: http://54.193.163.58:8877/f52358de0e3c929/reftest-analyzer.html#web=eq.log

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Success

Full output at http://54.241.84.105:8877/ea5d365cd68fe59/output.txt

Total script time: 18.29 mins

• Regression tests: Passed