Add option to keep prefix operators even in noop cases

[johannesboyne]

Some Excel users start formulas always with a =+ and thus, being able to have an option to keep even noop prefixes would be nice. E.g., in a formula like =+SUM(H8:H11)+-H12+(-H13).

[psalaets]

Thanks for contributing. I am reviewing your changes.

Are prefix operators expected to be kept everywhere or only at the front of the formula?

Example:

tokenize('SUM(1, +2)', {keepPrefixOperators: true})

returns

[
  { value: 'SUM', type: 'function', subtype: 'start' },
  { value: '1', type: 'operand', subtype: 'number' },
  { value: ',', type: 'argument', subtype: '' },
  // A
  { value: '2', type: 'operand', subtype: 'number' },
  { value: '', type: 'function', subtype: 'stop' }
]

Expected a token for the + prefix operator at A

[johannesboyne]

Happy to contribute!
From our perspective, we would always require all tokens the user used in the formula. Even if they are unused or not necessary. For example, the following would make a lot of sense to us:

tokenize('=+SUM(1, +2)', {keepPrefixOperators: true})
/* (somewhat pseudo return)
[
  { value: '=', type: 'operand', required_for_calculation: false},
  { value: 'SUM', type: 'function', subtype: 'start', required_for_calculation: true },
  { value: '1', type: 'operand', subtype: 'number', required_for_calculation: true },
  { value: ',', type: 'argument', subtype: '', required_for_calculation: true },
  { value: '+', type: 'operand', required_for_calculation: false},
  { value: '2', type: 'operand', subtype: 'number', required_for_calculation: true  },
  { value: '', type: 'function', subtype: 'stop', required_for_calculation: true  }
]
*/