`Base64URLNoPadding` (URL-safe Base64 without padding) was added

$(REF Base64URLNoPadding, std, base64) allows encoding/decoding without padding:

---
import std.base64 : Base64URLNoPadding;

ubyte[] data = [0x83, 0xd7, 0x30, 0x7b, 0xef];
assert(Base64URLNoPadding.encode(data) == "g9cwe-8");
assert(Base64URLNoPadding.decode("g9cwe-8") == data);
---
