add setting to enable environment variable insertion

[eleanorjboyd]

Feature request: add a setting called "python.terminal.injectEnvFile". This setting will default to false (not enabled). What it does is control if environment files (.env) files and the value of the python.envFile are injected into terminals as environment variables. see 6173146 for how the environment variables are injected and where that code is.

[eleanorjboyd]

so only create the TerminalEnvVarInjector if the setting is on but add a listener that will create the injector if the setting is turned on mid-activation