Add hot code reload

[iusildra]

Address this feature request, but for now, it should only works with sbt, I'll look to make it works with bloop too later. I adapted the implementation from vscode-java-debug to work with metals

The implementation on the debugger side is in this pr (not published yet)

There are a few inconsistencies left in my implementation on the VS Code extension:

• [x] If the setting scalaHotReloadOn is not ticked, the button still appear (we need to tick and untick to make it disappear)
• [x] If HCR is not supported by the debugger, it show a warning message "no changed classes found", which is not relevant

[iusildra]

It will need both upcoming versions of the debugger (for the implementation) and metals (for the sync with compilation) to be fully functional

Also, before merging I'll need to change the metals server's version before which HCR would not work to give proper information

[adpi2]

@tgodzik this PR is ready to be reviewed and merged.

It adds the lightning button in the debug tool bar to trigger a hot code reload.