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.

Userland/Libraries/LibC/grp.cpp

@@ -88,7 +88,7 @@ static bool parse_grpdb_entry(char* buffer, size_t buffer_size, struct group& gr
     auto& gid_string = parts[2];
     StringView members_string = parts[3];
 
-    auto gid = gid_string.to_uint();
+    auto gid = gid_string.to_number<gid_t>();
     if (!gid.has_value()) {
         warnln("parse_grpdb_entry(): Malformed GID on line {}", s_line_number);
         return false;