Add ZERO_U256 constant (#5829)

## Description

We provide a `ZERO_B256` but do not provide a `ZERO_U256` convenience
constant. This has added.

Closes #5828 

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] 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](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [ ] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

Author: bitzoic

sway-lib-std/src/constants.sw

@@ -33,6 +33,19 @@ pub const BASE_ASSET_ID: AssetId = AssetId::from(ZERO_B256);
 /// ```
 pub const ZERO_B256 = 0x0000000000000000000000000000000000000000000000000000000000000000;
 
+/// A u256 of zero value.
+///
+/// # Examples
+///
+/// ```sway
+/// use std::constants::ZERO_U256;
+///
+/// fn foo() {
+///     assert(ZERO_U256 == u256::from(0_u64));
+/// }
+/// ```
+pub const ZERO_U256 = 0x00u256;
+
 /// The default Sub Id for assets.
 ///
 /// # Examples