Everywhere: Use to_number<T> instead of to_{int,uint,float,double}

In a bunch of cases, this actually ends up simplifying the code as
to_number will handle something such as:

```
Optional<I> opt;
if constexpr (IsSigned<I>)
    opt = view.to_int<I>();
else
    opt = view.to_uint<I>();
```

For us.

The main goal here however is to have a single generic number conversion
API between all of the String classes.

Kernel/FileSystem/ProcFS/Inode.cpp

@@ -159,7 +159,7 @@ ErrorOr<NonnullRefPtr<Inode>> ProcFSInode::lookup_as_root_directory(StringView n
     if (name == "self"sv)
         return procfs().get_inode({ fsid(), 2 });
 
-    auto pid = name.to_uint<unsigned>();
+    auto pid = name.to_number<unsigned>();
     if (!pid.has_value())
         return ESRCH;
     auto actual_pid = pid.value();