Browser sniffing helpers should be deprecated by default; expose the UA parser for general use

Browser sniffing helpers should be deprecated by default; expose the UA parser for general use: a task in MiMo-V2.6-RL-oss: Agentic RL Environments (MiMo RL release). I'm using MooTools and ran into two related awkward bits around Browser: 1. The default build still encourages UA sniffing The…