add workaround for double.infinity

[wolfgio]

This pull request introduces support for handling infinity values in the BigDecimal class and includes several code simplifications and formatting improvements.

Support for Infinity Values:

• Added BigDecimalInfinity class to handle infinity values (lib/src/big_decimal_infinity.dart).
• Updated BigDecimal class to include a factory constructor for infinity values and modified the parse method to return BigDecimalInfinity when appropriate (lib/src/big_decimal.dart). [1] [2]

Code Simplifications:

• Removed the private constructor _ from BigDecimal and replaced all instances with the public constructor (lib/src/big_decimal.dart). [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Formatting Improvements:

• Reformatted code in BigDecimal class for better readability (lib/src/big_decimal.dart). [1] [2] [3]
• Reformatted test cases for better readability (test/big_decimal_test.dart). [1] [2] [3] [4] [5] [6] [7] [8]

Test Updates:

• Added test cases to verify the correct handling of infinity values (test/big_decimal_test.dart).

Dependency Updates:

• Updated exports and imports to include the new big_decimal_infinity.dart file (lib/big_decimal.dart, lib/src/big_decimal.dart, test/big_decimal_test.dart). [1] [2] [3]