Add JEP for sub-shells

[davidbrochart]

This PR introduces kernel sub-shells to allow for concurrent code execution. It is a JEP because it changes the kernel messaging protocol - these changes consist of (optional) additions.

Voting from @jupyter/software-steering-council

• @gabalafou
  • [x] Yes
  • [ ] No
  • [ ] Abstain
• @ivanov
  • [ ] Yes
  • [ ] No
  • [x] Abstain
• @JohanMabille
  • [x] Yes
  • [ ] No
  • [ ] Abstain
• @jtpio
  • [x] Yes
  • [ ] No
  • [ ] Abstain
• @marthacryan
  • [x] Yes
  • [ ] No
  • [ ] Abstain
• @martinRenou
  • [x] Yes
  • [ ] No
  • [ ] Abstain
• @minrk
  • [x] Yes
  • [ ] No
  • [ ] Abstain
• @rpwagner
  • [ ] Yes
  • [ ] No
  • [ ] Abstain
• @SylvainCorlay
  • [x] Yes
  • [ ] No
  • [ ] Abstain
• @vidartf
  • [ ] Yes
  • [ ] No
  • [ ] Abstain

[JohanMabille]

Let's extend the voting period to next Monday, since people may have been in holidays during the past few weeks.

[SylvainCorlay]

This JEP has been approved! 🎉

• Yes 7
• No 0
• Abstain 1
• Absence of vote 2