Improve handling of receiver types

[gonzojive]

Fixes https://github.com/fwcd/tree-sitter-kotlin/issues/153 by using receiver_type.

• I'm not too familiar with grammar-writing, so I'm unsure if my fixes here are ideal.
• function_type has been updated to more closely match the official grammar for functionType.
• [$.receiver_type], [$.receiver_type, $._type], have been added to the conflicts list. Adding these conflict entries made previously error-generating Kotlin inputs non-error-generating with sensible parse trees. It's unclear if this is the best strategy for specifying the grammar.

Draft mode because this also includes changes from https://github.com/fwcd/tree-sitter-kotlin/pull/152.