Resolve variables for executable path.

[TrevorSundberg]

If you use ${workspaceFolder} when specifying the clangTidy.executable path the variable is not resolved, for example ${workspaceFolder}/tools/llvm/bin/clang-tidy. This is useful for cases when clang-tidy is installed locally within the work-space. The list of variables that are usable are defined here.

I believe the change is done by calling resolve() here:

clang-tidy-linter/src/tidy.ts

Line 105 in 8681a1a

const childProcess = spawn(configuration.executable, args);

It looks like resolve() is defined here, but this is a base class and I'm unsure of the proper way to access it in vs-code extensions.