[Linter] Warn on pub let for dictionaries, arrays, capabilities, and structs with publicly callable functions

[rheaplex]

Cadence's access control may surprise the unwary in several ways:

• The content of pub let dictionaries and arrays can be mutated by anyone who can access them.
• pub let Capabilities can be copied and this may be undesirable.
• pub let struct member variables that expose any of the above, and/or have public functions, may surprise the unwary in any of these ways.

The Cadence documentation does describe these scenarios, but they can be serious enough that it is probably worth catching them in the compiler and emitting a warning.

(I thought there was a ticket for this, but I cannot locate it. 😺)