Do not remove/concat multiline strings in YAML

[pacien]

Description of the issue:

Multiline YAML strings (using the | modifier) are concatenated with \n
after going through Khard.

This is inconvenient when editing existing contacts.

It would be nice to write the YAML file in the human-friendly way keeping
those multiline | string values.

Example:

1. Create a new contact with khard new.

2. Set some multiline property, as suggested by the default template, such as:

   Note: |
     First line.
     Second line.

3. Save that contact.

4. Re-open that contact with khard edit.

5. Notice that the field is now shown for edition as:

   Note: "First line.\nSecond line."

Version info:

• Khard version 0.18.0
• Python 3.10.12
• NixOS 23.05

[lucc]

I think we serialize the data with the ruaml library. Maybe there is an option in the lib to do this.