fix: skip HMR websocket path when forwarding upgrades to user-defined proxies

[ajhyndman]

• This is a bugfix
• This is a feature
• This is a code refactor
• This is a test update
• This is a docs update
• This is a metadata update

For Bugs and Features; did you add new tests?

Please let me know what kind of test could be appropriate for this change.

Motivation / Use-Case

Both create-react-app and vue-cli-service have introduced workarounds for this, but it seems to me that this is always desirable behaviour for webpack-dev-server.

vuejs/vue-cli@72ba750
react/create-react-app#7444

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Andrew Hyndman (251a52f)

[alexander-akait]

Can you add test case and fix types?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.60%. Comparing base (b550a70) to head (686903d).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4316      +/-   ##
==========================================
+ Coverage   83.59%   83.60%   +0.01%     
==========================================
  Files          13       13              
  Lines        2054     2086      +32     
  Branches      756      773      +17     
==========================================
+ Hits         1717     1744      +27     
- Misses        303      306       +3     
- Partials       34       36       +2     

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