Highlighting doesn't support inline arithmetic expressions

[sclu1034]

As documented at https://beancount.github.io/docs/beancount_language_syntax.html#transactions, Beancount supports basic arithmetic in postings, like

2014-10-05 * "Costco" "Shopping for birthday"
  Liabilities:CreditCard:CapitalOne         -45.00          USD
  Assets:AccountsReceivable:John            ((40.00/3) + 5) USD
  Assets:AccountsReceivable:Michael         40.00/3         USD
  Expenses:Shopping

However, the highlighter assigns a scope of invalid.illegal.unrecognized.beancount to these expressions.

[a180285]

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