std.debug.leb128: Implement functions capable of writing LEB128 to some form of output

[Soupertonic]

While the leb128.zig file implements and provides functionality to read/parse/decode LEB128s, the file should also implement/provide functionality to write/encode LEB128s (into a buffer or stream).

Use Case

A simple but well known use case would be when dealing with Minecraft's protocol due to the fact it generally utilizing VarInts/VarLongs (LEB128) for numeric values.

[ifreund]

Implemented in #5588