Be able to use env file

[daravi]

It would be useful to be able to load an env file (similar to https://github.com/microsoft/vscode-go/issues/829). For example my package managment system (Conan) generates and en file. It would be easier for me to just point to this file than manually duplicate the content and update it whenever my dependencies change.

Note: it would probably be better if this can be a list of files. For example Conan can generate two environment files, both of which might be required to be activated at runtime.

[vadimcn]

I could use a PR for this one. Probably can be done similarly to the go extension. Though I think that "env" entries should override the ones in envfile. I am also not sure whether they've implemented file parsing correctly (or even what the format of that file is). Finally, please don't forget about Windows, where environment is case-insensitive. This class is designed to handle all that.

[vadimcn]

Implemented in v1.9.1