feat(orm): exclude function properties from `FieldName` type

[Char2sGu]

Summary of changes

Exclude function properties from FieldName type.

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[Char2sGu]

Types will not be computed when its argument is another generic type argument, thus we might have to use many type assertions in our code, which destroys the user experience:

function fn<Entity extends { username: string }>() {
  const field: FieldName<Entity> = "username"; // this throws a type error unless we use `as FieldName<Entity>`
}

Therefore this PR cannot be merged for now, except we find a solution some day.