native support for JSpecify

[fprochazka]

I would like to propose adding native support for https://jspecify.dev/docs/using/, so that instead of

dependencies {
    api("org.jspecify:jspecify:1.0.0")
}

strictNullCheck {
    packageInfo {
        imports.set(setOf("org.jspecify.annotations.NullMarked"))
        annotations.set(setOf("@NullMarked"))
    }
}

one could write

strictNullCheck {
    addJSpecify)
    packageInfo {
        useJSpecify()
    }
}