Add missing fields in keyed_element

[wetneb]

In struct instantiations, such as:

MyStruct {
    foo: 2,
    bar: 3,
}

the keyed elements are currently parsed as seq($.literal_element, ':', $.literal_element), meaning that as a consumer it is difficult to to distinguish the key from the value (given that they have the same type).

This PR proposes to add field names to this part of the grammar, so that they can be told apart.

[aryx]

LGTM

[wetneb]

Thanks for the lightning-fast review! I have fixed the linting error.