Introducing Abstract Syntax Tree Enum parsing support

[0x19]

This pull request introduces support for enums in the ast package. Enums provide a way to define a set of named values in Solidity contracts.

New Features and Enhancements

• Added EnumNode struct to represent enum definitions in the AST.
• Added EnumMemberNode struct to represent individual enum members in the AST.
• Implemented the Children method for EnumNode to return the enum members as child nodes.
• Added unit tests for the EnumNode struct to ensure correct functionality and JSON marshalling.

[coveralls]

Pull Request Test Coverage Report for Build 5448330585

• 18 of 23 (78.26%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 76.673%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ast/contract.go	2	4	50.0%
ast/enum.go	6	9	66.67%

Totals
Change from base Build 5448187494:	0.01%
Covered Lines:	871
Relevant Lines:	1136

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💛 - Coveralls