debug: investigate inline mode debug adapter

[hyangah]

From #23 (comment)

The debug adapter library allows three different modes of launching the debug adapter.
https://github.com/microsoft/vscode-mock-debug/blob/master/src/extension.ts#L57

In particular the inline mode is interesting. Explore and see if it's better for long term.

@eliben already has a prototype that wires the existing GoDebugSession in the inline mode.
https://github.com/eliben/vscode-go/commit/81042e0e813f781bb0e5d6eeb747834bb21d6d97

[eliben]

FWIW, the new DA (debugAdapter2) works in inline mode.

[hyangah]

Closing because this is no longer relevant. We will leave the legacy debug adapter as it is.
The new dlv dap adapter is integrated through an inline-mode thin adapter.