only use .g4 file
This calculator saves the value to be stored in the variable in the Map.
In addition, each statement can be separated by ;,
and the last statement does not have to be ;. (similar to Rust)
Supports multi-line calculations and storing result in variables.
a = 13;
a + 2;
1 + 3 * 2 - 3 * 715
-14
It also guarantees operator precedence.