[M3] eval command

[lisachenko]

eval is what makes the IDE's watch/immediate window work. It is currently
answered with DBGp error 4 (unimplemented).

Tasks

• Handle eval -i <tx> -- <base64(expression)> in
  ZDebug\Session\CommandDispatcher
• Evaluate in the context of the frame selected by -d (default 0), reusing
  the ConditionEvaluator machinery from the conditional-breakpoint ticket
• Return the result as a <property> via PropertySerializer
• Return DBGp error 206 (EVAL_FAILED) when the expression throws
• Advertise it in feature_get responses

Constraints

• Runs under the reentrancy latch (the eval'd code compiles EXT_STMT oplines
  and would otherwise re-enter the statement handler
• Never let a escape into the FFI callback
• Writing back to locals () is explicitly out of scope - see
  the limitations ticket
  EOF
  )