`std.StaticStringMap.initComptime` allows duplicate keys

### Zig Version

0.12.0

### Steps to Reproduce and Observed Behavior

```zig
pub fn main() !void {
    const Foo = enum { a, b };
    const NameMap = std.StaticStringMap(Foo).initComptime(.{
        .{ "c89", .b },
        .{ "c89", .a },
    });

    std.debug.print("{}\n", .{ NameMap.get("c89").? });
}
```

### Expected Behavior

Compile time error.