Add support for Coder.com (#714)

[deansheather]

Short description of what this resolves:

This adds support for the Coder.com VS Code Cloud IDE.

Changes proposed in this pull request:

• Added the property isCoderCom to Environment
• Added some setting overrides in a few places to settings that work for Coder.com
  • Force disable XDG home on Coder.com
  • Remove . in configSuffix
  • Force set the settings base path to /tmp, which resolves to /tmp/User
  • Force manual installation of extensions rather than using the CLI
• Fixed an unrelated TS lint issue so it would pass the linter

Fixes: #714

How Has This Been Tested?

I tested the plugin on regular VS Code on a Windows machine with a simple settings.json file and 3-4 extensions. The settings file was uploaded from the Windows box and then downloaded into the Coder.com VS Code Cloud IDE using the extension, which worked. The synced extension installations worked as well.

I also tested this in the other direction, uploading the config from Coder.com and downloading to a regular IDE and this also worked.

Screenshots (if appropriate):

n/a

Checklist:

• I have read the contribution guidelines.
• My change requires a change to the documentation and Github Wiki.
• I have updated the documentation and Wiki accordingly.