Support string literal dependency notation for add calls

[madisp]

This currently fails with parsing:

dependencies {
  add("myconfig", "com.example:foo:1.2.3")
}

parse error:

1: extraneous input '"' expecting {'file(', 'files(', 'project', ID}, 2: extraneous input '"' expecting {'}', ID}

[autonomousapps]

Thanks for the issue.

[madisp]

NOTE: as a workaround, version catalog references are ok, e.g. this will work:

dependencies {
  add("myconfig", libs.foo)
}