Added nav-mesh support

**Changes**:
* Several TF/NextBot-specific features have been `#ifdef`'d out of the nav-mesh code.
* Added a `nav_mesh.vpc` script that is included by `server_hl2mp.vpc` if `$SOURCESDK` is defined.
* Nav edit mode no longer uses `NDEBUG_PERSIST_TILL_NEXT_SERVER` as the overlay duration. This is too long and causes overlapping, increased fillrate and flickering. All durations have been set to 0 (i.e., until the next frame).

**Note to Valve devs**: when merging this pull request into your own codebase, be sure to add the `USE_NAV_MESH` pre-processor definition to your TF VPC script (if you're not including `nav_mesh.vpc`). This is to ensure the nav-mesh code gets built in `gameinterface.cpp` (see the changes for more info).