Scanner.getIncrement

private bool getIncrement(ref dchar c, ref int increment, const Mark startMark) pure @safe

Get increment indicator, if detected. Return false otherwise.

Used in scanBlockScalarIndicators.

`c`	The character that may be an increment indicator. If an increment indicator is detected, this will be updated to the next character in the Reader.
`increment`	Write the increment value here, if detected.
`startMark`	Mark for error messages.