Allow decimal/binary/octal u256 literals

[saimeunt]

Description

This PR removes the constraint on u256 literals being only expressed in hexadecimal.
The constraint has been removed and the code slightly refactored to remove the unnecessary radix variable in the lex_int_lit function in token.rs.
The corresponding error LexErrorKind::U256NotInHex has been removed from the codebasen as well as the should_fail test looking to raise this error.

The u256 testing suite has been augmented with additional tests to make sure we can express u256 literals in decimal, binary and octal.

Closes #5456

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[IGI-111]

Looks good to me!

[IGI-111]

looks like the one CI issue is unrelated to this PR @JoshuaBatty any idea why that utils test is failing?

[IGI-111]

Looks like it's just flaky