Provide documentation / function to create Decimals

[theCapypara]

Currently the crate has no documentation or function to create Decimals, which are internally just Strings. From what I understand, they are the same as the Admin API Decimals (https://shopify.dev/docs/api/admin-graphql/2023-01/scalars/Decimal).

It would probably be useful and eliminate sources of errors, if the crate had a utility function to format Rusts built-in number types as Decimals and/or to implement a proper Decimal type or use a proper Decimal type of another crate.
At the very least, there should be some documentation in the crate on what values Decimals should have (see link to Admin API).

[theCapypara]

To add to this, a proper Decimal type would also allow for comparing Decimals and doing calculations with it.

[adampetro]

A decimal helper type was introduced in #34 and released in version 0.4.0. Feel free to reopen if you have any issues with it