Figure out how to make Runfiles library accessible to external workspaces

Currently an external workspace will have to do some pathing to figure out how to reference the csproj for the runfiles project.

[x] Set BazelExternal when building to allow convenient csproj references to any projects in the external directory via $(BazelExternal)
[x] Add BazelExternal to Bazel.props for IDE integration
[x] Support $(BazelExternal) path references in csproj files in gazelle
[ ] Add e2e test that accesses runfiles

samhowes / rules_msbuild