feat: use standard fetch API when using node >=18

[CamilleLetavernier]

fixes #12775

What it does

• remove the dependency to 'http(s)-proxy-agent'
• replace it with Node fetch + undici (for Proxy)
• replace the deprecated parseurl() with new URL()

How to test

• Test some commands that depend on the NodeRequestService / Proxy, e.g.:
  • In Command Palette, check that "Configure Display Language" works properly
  • In Extensions, search for any OpenVSX extension and verify that extensions can be found/downloaded

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines
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