wrong macro expansion for $dir in windows ! (includes backward slash)

## Environment data
VS Code version: 1.19.1
Python Extension version: 3.5.2
Python Version: 3.5
OS and version: Windows 8.1

## Actual behavior
in settings.json , when we use `$dir ` in any settings , it takes backward slash as separator and becoz of that "path1\path2\path3\" , since it includes a backward slash at end of path , that end quotes " gets ignored and it throws me a file not found error .

## Expected behavior
`$dir` should resolve to `"path1\\path2\\path3\\"` or `"path1/path2/path3/"` (with forward slashes)

## Steps to reproduce:
Put this in settings.json

```
"code-runner.executorMap": {
"cpp": "cd $dir "
}
```

Reproduce the issue on Windows using settings.json and the code-runner.executorMap entry shown in the report. Trace how `$dir` is expanded before the command is executed, especially when the resolved path ends with a backslash. Done means the generated command remains valid and uses escaped backslashes or forward slashes without causing a file-not-found error.

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