* Generate Visual Studio projects using Premake

This change adds a `premake5.lua` file that allows us to generate our Visual Studio solution using Premake 5 (https://premake.github.io/).
The existing Visual Studio solution/projects are now replaced with the Premake-generated ones, and project contributors will be expected to update these by running premake after adding/removing files.

I have *not* changed the Linux `Makefile` build at all, because that file is also used for things like running our tests, so that clobbering it with a premake-generated `Makefile` would break our continuous testing.
Hopefully future changes can switch to a generated `Makefile` and perhaps even add an XCode project as well.

Notes:

* The `build/slang-build.props` file is no longer needed/used, so it has been removed.

* The `slang-eval-test` test fixture wasn't following our naming conventions for its directory path, so it was updated to streamline the Premake build configuration work. This required changes to the `Makefile` as well

* Some seemingly unncessary preprocessor definitions that were specified for `core` and `slang-glslang` have been dropped. We will see if anything breaks from that.

* Possible fixup for Premake vpath issue

Premake's `vpath` feature seems to be nondeterministic about the order it applies filters (because Lua isn't deterministic about the order of entries in a key/value table), and as a result we can end up in a weird case where it decides that a `foo.cpp.h` file matches the `**.cpp` filter (I'm not sure why) before it tests against the `**.h` filter.

This change uses an (undocumented) Premake facility to set `vpath` using a list of singleton tables, which seems to fix the order in which things get tested.

* Remove support for "single-file" build of Slang

The `hello` example was the only bit of code that uses the "single-file" way of building Slang, and this had already run up against limitations of the Visual Studio compilers in its Debug|x64 build.
Rather than mess with Premake to make it pass through the `/bigobj` linker flag that is needed to work around the issue, it makes more sense just to stop using/supporting the feature since we wouldn't want users to depend on it anyway (our documentation no longer refers to it).

While I was at it I went ahead and made sure that the `SLANG_DYNAMIC` flag doesn't need to be set manually, so that instead there is a non-default `SLANG_STATIC` option (not that we have a static-library build of Slang at the moment).