Allow EnvironmentVariableCollection API to apply changes via shell integration

[Tyriar]

A problem the Python extension has is that it sets environment variables but they get overridden during shell startup scripts. This issue would allow adding an option to the variable mutator for the timing a particular environment variable is applied.

[karrtikr]

@Tyriar FYI none of the integration scripts work if VSCODE_ENV_* variable is set, each shell fails with its own type of error.

[sandy081]

Moving this to next milestone as this is a feature request and we are late in the release cycle to make this happen.