Uppercase, incomplete accounts result in `ERROR` nodes

[aldur]

The following example:

2023-10-01 * "payee example" "narration exampl"
  A

Produces the following tree:

(file ; [0, 0] - [2, 0]
  (transaction ; [0, 0] - [1, 0]
    date: (date) ; [0, 0] - [0, 10]
    txn: (txn) ; [0, 11] - [0, 12]
    payee: (payee) ; [0, 13] - [0, 28]
    narration: (narration)) ; [0, 29] - [0, 47]
  (ERROR ; [1, 0] - [1, 3]
    (ERROR))) ; [1, 2] - [1, 3]

Turning the initial lowercase results into an identifier instead:

2023-10-01 * "payee example" "narration exampl"
  a

(file ; [0, 0] - [2, 0]
  (transaction ; [0, 0] - [1, 0]
    date: (date) ; [0, 0] - [0, 10]
    txn: (txn) ; [0, 11] - [0, 12]
    payee: (payee) ; [0, 13] - [0, 28]
    narration: (narration)) ; [0, 29] - [0, 47]
  (ERROR ; [1, 0] - [1, 3]
    (identifier))) ; [1, 2] - [1, 3]

This impacts LSP completion (which relies on TreeSitter), because uppercase letters won't get account completion. Ref: https://github.com/polarmutex/beancount-language-server/issues/477.