debug: warn users if they use the launch/attach configuration properties unsupported by dlv-dap or legacy mode

[hyangah]

Currently we are also reading the default configuration from settings.json, so should check if users set it from settings.json or not.
If it's too complex, improving the documentation may be sufficient.
And I hope the existence of those attributes doesn't stop or prevent debugging.

cc @suzmue @polinasok

[gopherbot]

Change https://golang.org/cl/315149 mentions this issue: src/goDebugConfiguration.ts: show warning for dlv-dap with dlvLoadConfig

[gopherbot]

Change https://golang.org/cl/316470 mentions this issue: src/goDebugConfiguration.ts: disable warning about using 'cwd' in dap