Add support for Decimal data type

[Royal-lobster]

Fixes #152

Add support for serialization of Decimal data type.

• src/transformer.ts

  • Import the Decimal class from the decimal.js library.
  • Add a transformation rule for the Decimal data type.
  • Update the compositeRules array to include the new Decimal transformation rule.

• src/index.ts

  • Register a custom transformer for the Decimal data type in the SuperJSON class constructor.

• src/index.test.ts

  • Update the test case for Decimal.js to use the built-in transformer.

[Skn0tt]

Hi there, thanks for your contribution! Adding a polyfill as a dependency to SuperJSON isn't the right route for this, it'll make all users of SuperJSON download it, even though very few use it. I'm open to adding support for this once https://github.com/tc39/proposal-decimal advances.