Report only files specified by mypy config

[karthiknadig]

Add a setting to check only a subset of files in the workspace folder. The actual list of files is specified in the files mypy configuration.

We omit path argument to mypy, so that it uses files defined in the configuration.

How to manually verify the feature:

1. Create a project with 2 python files: foo.py and bar.py, each containing typing error.
2. In the project root create a pyproject.toml with the following contents:

[tool.mypy]
files = ["foo.py"]

3. In the project configuration, set mypy-type-checker.reportingScope to custom.
4. Open foo.py, it should have a typing error highlighted
5. Open bar.py, it should NOT have typing errors highlighted