Fix Emitter to handle comments between a mapping key and a mapping/sequence value

[ghost]

This PR fixes #812

TL;DR the Emitter doesn't handle comment events that are emitted between a mapping key and a mapping value of the mapping or sequence type. So parsing and regenerate the following yaml will fail for example:

hello:
  # comment
  - world

becomes

hello
# this is a comment
:
- world

This PR moves the writing of the : symbol to be inside the block mapping key emission. Previously it was at the beginning of the block mapping value emission. If the mapping key is prefixed with a ? (explicit form), however, the : is still kept at the mapping value to preserve the existing behaviour, since in the explicit form, the comment does not break YAML syntax. That is,

? hello
# this is a comment
:
- world

is valid.

[aaubry]

This feature has been released in version 13.1.1.