Add support for `-embed-dir` or "--embed-dir" in compilerArgs

[sean-mcmanus]

gcc and clang both have an --embed-dir= argument that essentially works as an include dir but for the #embed directive. It doesn't appear there is any way to set this so intellisense will properly locate files.

Originally posted by @Flat in #13705

[sean-mcmanus]

It seems like https://github.com/microsoft/vscode-cpptools-api and https://github.com/microsoft/vscode-cmake-tools may need to be updated too for the new embedPath.

...or maybe we'll just support it via compilerArgs with "-embed-dir" (gcc) or "--embed-dir" (clang) (or at least initially)?