chore: improve dfx start messages.

[vincent-dfinity]

Description

1. Improve dfx start messages to let users know what to do next clearly.
2. Add PostStart actor which will be notified by PocketIcProxy actor to output the messages in the right order.
3. Set the original_background environment variable which will be used by the second start, as for now with --background we will run the dfx start again without --background option, but we need its value in PostStart actor.

Fixes # (issue)

SDK-1844

How Has This Been Tested?

Manually tests.

$ dfx start
Running dfx start for version 0.23.0-beta.3+rev34.dirty-11793309
Using the default configuration for the local shared network.
Initialized replica.
Initialized HTTP gateway.
Replica API running on 127.0.0.1:4943
The dfx server is running.
You can start a new terminal to continue developing, or quit with 'Ctrl-C'.

$ dfx start --background
Running dfx start for version 0.23.0-beta.3+rev34.dirty-11793309
Using the default configuration for the local shared network.
Initialized replica.
Initialized HTTP gateway.
Replica API running on 127.0.0.1:4943
The dfx server is running in the background.

Also add e2e test for dfx start --background, not sure how to add test for dfx start...

Checklist:

• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly.
• I have made corresponding changes to the documentation.

[marydwyer]

Because new user often don't realize that they successfully ran dfx start and need to open a new terminal, I recommend making it even more clear.

For dfx start:
✅ Success! The dfx server is running.
➕ You must open a new terminal to continue developing. If you'd prefer to stop, quit with 'Ctrl-C'.

For dfx start --background:
✅ Success! The dfx server is running in the background.