Add VERSION to autoloads (#869)

Makes `Phlex::VERSION` automatically available, without having to
require it manually.

Re:
https://discord.com/channels/1082611227827638303/1082612756676620318/1343897758045573140

```
$ bundle console
>> require "./lib/phlex"
=> false
>> Phlex::VERSION
=> "2.0.2"
```