I understand that there are implementation difficulties in retrofitting single quotes for multiline strings, but I think it’s worth it if what we get is a single " syntax to cover all string literal use cases.
With years, even triple quotes (""") could be deprecated, continuing the effort to make Scala 3 a smaller language.
Please don’t introduce another ''' syntax 
. I’d rather live with the current
"""{
| "foo": "bar"
|}""".stripMargin
than have yet another syntax (which would use a character (') currently related to Char, not String).
About the argument that it’s normal for other languages to define several different ways to create multiline strings, I’d cite the Zen of Nim’s first principle:
Copying bad design is not good design.