isDecimal function
Checks if the string represents a decimal number.
Accepts an optional sign followed by digits with an optional fractional
part (e.g. 1, -1.5, .5, +10.0).
Example:
isDecimal('1.5'); // true
isDecimal('-0.25'); // true
isDecimal('.5'); // true
isDecimal('1.'); // false
isDecimal('abc'); // false
Implementation
bool isDecimal(String str) => _isDecimal(str);