Implement "match" control flow structure

[alexdovzhanyn]

Add a pattern matching control flow structure, as shown below:

match (variable) {
  when :ok >> something()
  when :error >> something_else()
  default >> some_default_thing()
}

• Update the lexer to recognize match, when, default, and >> as language keywords and operators, respectively.
• Update the parser to recognize the match statement.
• Modify the AST to accommodate the match statement and its branches. This can be modeled with the ControlFlowNode, though it may need to be modified to add flexibility.
• Implement the necessary changes in the type checker and code generator.
• Ensure that match can handle various patterns and data types.
• Ensure that typechecking is honored during the pattern match