feat(parser): allow syntax sugar with `const`

[kshku]

Currently

const fn FunctionName() {}

const type TypeName {}

const type TypeName TypeInstName {}

is not possible, which should be equivalent to

const FunctionName = fn() {}
const TypeName = type {}
const TypeInstName = type TypeName {}

TODO: What to do with const before the value? (like const fn () {}, const type {}, etc?) Should they throw error similar to how const 5 is not allowed?