Updates to LDM Reviewed labels in this repository

We have begun reviewing issues (sorry this has been delayed). As part of this, we have added 4 new labels.

LDM In Process: LDM review is in progress
LDM Considering: LDM reviewed and think this has merit
LDM No Plans: LDM reviewed and this feature is unlikely to move forward in the foreseeable future
LDM Rejected: LDM reviewed and rejected

To be honest, we struggled with the descriptions. The initial "LDM No Plans" was not sufficiently clear that we probably won't do it, at least not in the near term. Some of these are good ideas, they just don't align with our current review guidelines, which are in the [6/13 meeting notes.](https://github.com/dotnet/vblang/blob/master/meetings/2018/vbldm-notes-2018.06.13.md), which also includes some of the guidelines we expect to use in reviewing issues.