Modify shell prompt when inside a snap

[Michael Hall]

Reason:

When debugging a snap with "snap run --shell" it's easy to forget when you are inside of the snap's environment and when you are not.

Solution:

When entering a shell inside a snap, add the snap's name to the shell prompt. Something like:
export PS1=($SNAPNAME)$PS1

[Sergio Schvezov (sergiusens)]

this change will make things using the shell look not so nice, like asciinema.

[John Lenton (chipaca)]

@sergio, we _could_ do it just for 'snap run --shell'.