parser: cleanup and simplify language parsing

[ttytm]

🤖[deprecated] Generated by Copilot at 38e6033

Refactor the parsing of function, struct, and interface languages in V. Use a common parse_language method in fn.v and struct.v to avoid duplication and simplify the code. Replace mut variables and multiple if statements with if expressions for readability and consistency.

🤖[deprecated] Generated by Copilot at 38e6033

• Refactor the parsing of the language of a function, type, struct, or interface declaration into a separate method parse_language that returns an ast.Language enum value (link, link, link)
• Simplify the assignment of the function name based on the language using an if expression instead of a mut variable and multiple if statements in vlib/v/parser/fn.v (link)
• Move the parsing of the language of a type declaration from vlib/v/parser/parser.v to vlib/v/parser/fn.v and use an if expression instead of a mut variable and multiple if statements (link)
• Remove the redundant code that consumes the dot token after the language name in vlib/v/parser/fn.v (link)

[spytheman]

Excellent work.