Add Eq implementation for String (FuelLabs#5055)

## Description

The `Eq` implementation seems to have been missing since moving the
`String` type from sway-libs to the standard library.

## 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).
- [x] 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.

Co-authored-by: bitzoic <[email protected]>

sway-lib-std/src/string.sw

@@ -259,6 +259,12 @@ impl From<raw_slice> for String {
     }
 }
 
+impl Eq for String {
+    fn eq(self, other: Self) -> bool {
+        self.bytes == other.bytes
+    }
+}
+
 // Tests
 
 #[test]
@@ -479,3 +485,13 @@ fn string_test_with_capacity() {
 
     assert(string.capacity() == 4);
 }
+
+#[test]
+fn string_test_equal() {
+    let string1 = String::from_ascii_str("fuel");
+    let string2 = String::from_ascii_str("fuel");
+    let string3 = String::from_ascii_str("blazingly fast");
+
+    assert(string1 == string2);
+    assert(string1 != string3);
+}