Add base32 encode/decode to gleam_stdlib

[xikaos]

Context

While playing with a TOTP generator/validator, I realized Gleam does not have native base32 encoding/decoding support.

That means gleamlings have two options:

1. Implement a native Gleam base32 encoder/decoder (ref).
2. Leverage existing JS/Erlang code trought FFI and expose it to our modules (ref).

Given that base32 is specially useful for:

• TOTP/HOTP secrets and otpauth:// URIs
• DNS- and filename-friendly encodings
• Text-only or case-insensitive channels

I propose adding base32_encode() and base32_decode() as functions in the Gleam stdlib.

Motivations

• Base32 (RFC 4648) is a standard peer to base16 and base64.
• Eliminate third-party dependencies when base16/base64 are already part of stdlib.
• Consistent cross-target availability (Erlang and JavaScript).

Proposed API

Public Gleam functions in bit_array:

• base32_encode(input: BitArray, padding: Bool) -> String
• base32_decode(encoded: String) -> Result(BitArray, Nil)

Examples

import gleam/bit_array

// Encode with/without padding
bit_array.base32_encode(<<"foobar":utf8>>, True)  // "MZXW6YTBOI======"
bit_array.base32_encode(<<"foobar":utf8>>, False) // "MZXW6YTBOI"

// Decode (case-insensitive, padding optional)
bit_array.base32_decode("MZXW6YTBOI======") // Ok(<<"foobar":utf8>>)
bit_array.base32_decode("mzxw6ytboi")