Implement Public/Private Access Control for Capsule Elements

[alexdovzhanyn]

Investigate the syntax for addition of public/private elements in a capsule (e.g., private functions and variables). Elements not explicitly marked as private should be considered public by default.

Subtasks:

• Research common syntax conventions for public/private access control in programming languages.
• Define a syntax for specifying public/private elements in a capsule.
• Implement parsing logic for the new syntax in the lexer and parser.
• Update AST nodes to support public/private access control.
• Ensure the default access level for elements is public unless explicitly marked as private.
• Write unit tests to verify the correct handling of public/private elements.