add support for multiplying parentheses

[juca1331]

Hello, the package didn't have the capability to perform mathematical operations with parentheses, so I decided to add this functionality within the parse method of the Parser class. To enable this functionality, it's necessary to set the 'multiplyWithParentheses' attribute to true in the parse method.

[fkleon]

Hi @juca1331, thanks for your pull request!

If I'm understand this change correctly, it will allow the parser to interpret a missing operator between parenthesis to be an implicit multiplication? To be honest, the code is a bit hard to follow. If that understanding is correct, I'm not sure that string manipulation on the input expression string is the best way to achieve this. Such functionality would be better implemented during or after the tokenization step within the parser.

A few further notes:

• Could you please update this PR to only include relevant changes? There is quite a lot of noise, but I think the relevant files are only parser.dart and the CHANGELOG file.
• Any new functionality requires unit tests to be added. This proves that the code is correct, ensures it is maintainable going forward, and also helps readers to understand how the feature is supposed to work.