fix: allow zero-padding for pointer formatting (fixes #2025)

[Muptezel58]

Problem

Using the 0 (zero-pad) flag with pointer arguments currently raises:
format specifier requires numeric argument

Root Cause

In include/fmt/base.h, the case '0': handler only allows
is_arithmetic_type, which excludes pointer_type.

Fix

Extended the check to also permit pointer_type:

// Before:
if (!is_arithmetic_type(arg_type))

// After:
if (!is_arithmetic_type(arg_type) && !in(arg_type, pointer_set))

Test

void* ptr = (void*)0x2993;
fmt::format("{:018}", ptr); // Previously threw, now returns: 000000000000x2993

Closes #2025