feat!: Use tomelr for TOML front-matter generation

[kaushalmodi]

Move YAML generation logic to ox-hugo-deprecated.el.

NOTE: At some point, YAML generation support will be removed entirely.

Minor breaking changes:

• Empty strings are not considered as nil.
• Nil is not considered as TOML false. A key with nil value means that it doesn't need to be exported to TOML.

Cosmetic changes:

• Multi-line strings are auto-generated if the string contains double quotes.