Feature request: read `minimum_zig_version` field of build.zig.zon

Thanks for making this awesome tool! I love how simple it is to install and use. What do you think about a feature like this? 1. when `zigup` is run within a folder that contains a `build.zig.zon`, zigup will install the version of the compiler specified by the `minimum_zig_version` field. if running `zigup` with no arguments does too much in this case, then perhaps it could be: * `zigup zon` * `zigup here` * `zigup this` * `zigup cwd`