feat: introduce --abs-proxy-base-path that allows app proxying while code-server is not server at the root

[rafaelnferreira]

What it is

• Allows proxy traffic to be properly routed when code-server is served under a path.

How

• Introduces a new switch abs-proxy-base-path.
• When specified, absproxy requests will be forwarded using the value as prefix

Example

• Code server is hosted at: my-codeserver.com/user/123
• React App is started at port 8080 under PUBLIC_PATH /user/123
• abs-proxy-base-path set to /user/123
• A GET request to my-codeserver.com/user/123/absproxy/8080/app will properly reach the app.

Fixes #6770

[code-asher]

This is great thank you for implementing!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6958   +/-   ##
=======================================
  Coverage        ?   72.63%           
=======================================
  Files           ?       31           
  Lines           ?     1904           
  Branches        ?      412           
=======================================
  Hits            ?     1383           
  Misses          ?      441           
  Partials        ?       80           

Files	Coverage Δ
src/node/cli.ts	90.90% <ø> (ø)
src/node/routes/index.ts	76.41% <ø> (ø)
src/node/routes/pathProxy.ts	82.14% <100.00%> (ø)

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[code-asher]

Looks like our formatter is being a bit weird, let me fix it real quick

Edit: actually not sure what is going on, CI seems to format a little differently compared to local...I will undo the indentation for now and figure it out later.