Allow properties with context receivers in interfaces to omit declaring accessors

[drawers]

Closes #1525

• Move check for getters and setters on properties with context receivers into TypeSpec.kt
• Disallow abstract properties in non-abstract types

[Egorand]

Thanks!

Too bad we can't perform all of these checks when creating the PropertySpec, but it seems that we don't have enough context until we add that property to a TypeSpec. We can perform part of the checks inside PropertySpec's constructor (i.e. has an abstract modifier, context receivers and no getters/setters - fail), but not sure whether there's value in splitting these checks between multiple types and thus complicating logic.