Always escape when writing pubspec.yaml's 'description' field.

[matanlurey]

Closes #80013.

Before:

$ flutter create test1 --description "a: b"
Creating project test1...
Error detected in pubspec.yaml:
Error on line 2, column 15: Mapping values are not allowed here. Did you miss a colon earlier?
  ╷
2 │ description: a: b
  │               ^
  ╵
Please correct the pubspec.yaml file at /Users/matan/Developer/scratch/test1/pubspec.yaml

After:

$ flutter create test1 --description "a: b"
Creating project test1...
Resolving dependencies in test1... 
Got dependencies in test1.
Wrote 129 files.

All done!
You can find general documentation for Flutter at: https://docs.flutter.dev/
Detailed API documentation is available at: https://api.flutter.dev/
If you prefer video documentation, consider: https://www.youtube.com/c/flutterdev

In order to run your application, type:

  $ cd test1
  $ flutter run

Your application code is in test1/lib/main.dart.

---

It's worth noting that this always escapes a non-empty project description, which means that descriptions that were not previously wrapped in "s' will be. I'm not sure how worth it is to do a conditional escape (i.e. only escape if not escaping would cause a problem), but willing to change.

Side-note: I had no idea where to list this test in the (very large) create_test.dart, so I did my best :)

[christopherfujino]

LGTM

[andrewkolos]

LGTM

[matanlurey]

Thanks!

[christopherfujino]

@matanlurey btw, you needed two org member approvals and for one of them to add the autosubmit label because you are not a flutter GitHub org member. Would you like me to nominate you for membership? The process is: https://github.com/flutter/flutter/wiki/Contributor-access