Remove ZigString.Slice.clone(...) (#21410)

### What does this PR do?

Removes `ZigString.Slice.clone(...)` and replaces all of its usages with
`.cloneIfNeeded(...)` which is what it did anyway (why did this alias
exist in the first place?)

Anyone reading code that sees `.clone(...)` would expect it to clone the
underlying string. This makes it _extremely_ easy to write code which
looks okay but actually results in a use-after-free:

```zig
const out: []const u8 = out: {
    const string = bun.String.cloneUTF8("hello friends!");
    defer string.deref();
    const utf8_slice = string.toUTF8(bun.default_allocator);
    defer utf8_slice.deinit();

    // doesn't actually clone
    const cloned = utf8_slice.clone(bun.default_allocator) catch bun.outOfMemory();
    break :out cloned.slice();
};

std.debug.print("Use after free: {s}!\n", .{out});
```
(This is a simplification of an actual example from the codebase)

src/string.zig

@@ -89,7 +89,7 @@ pub const String = extern struct {
                     }
                 }
 
-                return .{ @constCast((try utf8_slice.clone(allocator)).slice()), true };
+                return .{ @constCast((try utf8_slice.cloneIfNeeded(allocator)).slice()), true };
             },
             .StaticZigString => return .{ try this.value.StaticZigString.toOwnedSlice(allocator), false },
             else => return .{ &[_]u8{}, false },