Support base 2-36 in bigint toString() functions

[linusg]

• std.fmt.formatInt() supports it:

  zig/lib/std/fmt.zig

  Line 1178 in c172877

  assert(base >= 2);

  zig/lib/std/fmt.zig

  Lines 1213 to 1219 in c172877

  	while (true) {
  	const digit = a % base;
  	index -= 1;
  	buf[index] = digitToChar(@intCast(digit), case);
  	a /= base;
  	if (a == 0) break;
  	}

• std.math.big.int doesn't - it also uses digitToChar() so this limitation seems arbitrary.

  zig/lib/std/math/big/int.zig

  Lines 2298 to 2300 in c172877

  	pub fn toString(self: Const, string: []u8, base: u8, case: std.fmt.Case, limbs_buffer: []Limb) usize {
  	assert(base >= 2);
  	assert(base <= 16);

  zig/lib/std/math/big/int.zig

  Lines 2363 to 2368 in c172877

  	while (i < digits_per_limb) : (i += 1) {
  	const ch = std.fmt.digitToChar(@as(u8, @intCast(r_word % base)), case);
  	r_word /= base;
  	string[digits_len] = ch;
  	digits_len += 1;
  	}

Use case: https://codeberg.org/kiesel-js/kiesel/issues/68