fix(@lingui/loader): accept webpack 5.x as a peer dependency

[jesec]

npm 7 mandates matching peer dependencies. See RFC 25.

Fixes "Could not resolve dependency: npm ERR! peer webpack@"^4.0.0" from @lingui/loader"

@lingui/loader works perfectly fine with Webpack 5.x and a medium-size project.

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[codecov]

Codecov Report

Merging #971 (041d7f6) into main (a43b71c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #971   +/-   ##
=======================================
  Coverage   83.30%   83.30%           
=======================================
  Files          53       53           
  Lines        1605     1605           
  Branches      430      430           
=======================================
  Hits         1337     1337           
  Misses        156      156           
  Partials      112      112           

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[semoal]

Great mate, we'll release it as soon as possible