FIR checker: introduce top-level property checkers

Also, refactor property initializer checking so that those
newly added property checkers as well as member property checker
can share the same logic

compiler/testData/diagnostics/tests/sourceCompatibility/noMultiplatformProjects.fir.kt

@@ -1,5 +1,5 @@
 expect fun foo1()
-expect val bar1 = 42
+expect val bar1 = <!EXPECTED_PROPERTY_INITIALIZER!>42<!>
 expect class Baz1
 
 actual fun foo2() = 42