Set the user-installed `bin` directory when "activating" a terminal

[brettcannon]

E.g. if you choose your system-installed Python and then run Create Terminal, we should make sure that .local/bin (or whatever the appropriately directory is for your OS) is on PATH.

[DonJayamanne]

Sounds like a resurrection of #900

[DonJayamanne]

I've implemented a solution in my personal extension and the solution is as follows:

Create a temporary folder

• Add the files python.cmd into the above folder if on windows, else add python
  the contents of the above file is to launch the actual python file (hence the file created will be specific to each python enviromment), same with the folder
• Update the Path variable (either when you create or update the PATH variable in existing terminals) in the terminal to include the above directory
• Now when you run python it will run the right executable.