Emscripten detection does not work on windows

### Brief Issue Summary

Emscripten detection does not work on windows

### Expected:

When looking for kits, Emscripten should be detected when `EMSCRIPTEN` or `cmake.emscriptenSearchDirs` is correctly set to the emscripten folder.
(For example, `C:\emsdk\upstream\emscripten\`)

### Apparent Behavior:

Emscripten is not detected

### CMake Tools Log

Nothing related

### Developer Tools Log

Nothing related

### Platform and Versions

- **Operating System**: Windows
- **CMake Version**: 3.15.2
- **VSCode Version**: 1.51.1
- **CMake Tools Extension Version**: 1.5.2
- **Compiler/Toolchain**: Emscripten latest (releases-upstream-574ad04affb82cc36a32dd89b2a87bea4fb30eba-64bit)

### Other Notes/Information

As a side note, emsdk_env sets `EMSCRIPTEN_ROOT`, it should probably be checked too.

No source file, test, or entry point is named. Start by tracing kit detection on Windows and how it reads EMSCRIPTEN and cmake.emscriptenSearchDirs; done means the Emscripten folder is detected when either setting points to it, with EMSCRIPTEN_ROOT considered if appropriate.

Written by the indexing model from the issue text.