feat(frontends/lean): add SHOW and SLEEP debugging support commands, fixes worker interrupted bug, and LEAN_SERVER_DIAGNOSTIC compilation mode

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/server): in valid line tracking, add 'VALID' command similar to show, but marks invalid lines with a '*'

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(tests/lean/interactive): test output

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>