[Enhancement] Distinguish keyword arguments from type annotations

[yhk1038]

Currently, there is no way to distinguish between keyword arguments and type annotations.

# Type annotation: parameter x has type String
def foo(x: String)
  x
end

# Keyword argument: parameter x has default value "default"
def foo(x: "default")
  x
end

Both syntaxes use the same param: value format, making it impossible to differentiate between them.

This requires syntax design review.