Added NatSpec support for enum value definitions

[lilyanB]

Component

Forge

Describe the feature you would like

Currently, solidity doesn't support @param for enums (according to the natspec format). So when generating the documentation with forge doc, there is no way to document each enum variant.
The suggestion is to define a @custom:variant item to describe each enum variant.
Foundry already does something similar with unnamed function params ( see here ).

Additional context

Example of result:

/**
 * @dev Enum representing the status of a transaction
* @custom:variant Pending the transaction is pending
* @custom:variant Done the transaction is done
* @custom:variant Revert the transaction is revert
*/
enum Transaction {
    Pending,
    Done,
    Revert
}