Allow passing custom parameters to ispc compiler in VS code extension

While you can configure the target OS, target CPU, and general target for the ispc compiler in VS Code settings, there is currently no way to pass custom parameters (ex. `--enable-llvm-intrinsics`). The "compiler path" option also doesn't seem to work if flags are passed in there.

As a workaround, I'm creating a shell script which redirects options and setting the compiler path to that, but a built-in option would be great.

`ispc.sh`:
```bash
#!/bin/bash
# hack for VS code ISPC extension: extension has no way to set custom compiler flags,
# and ispc.compilerPath needs to be an executable, so I reroute it this way.
ispc --enable-llvm-intrinsics "$@"
```