Allow apps to specify mod version requirements (#798)

* Allow apps to specify mod version requirements

* Allow a function in requires field and move to `readConfig()`

* Add `server_mods` to system_info()

* Add prefixes and default to `=`

* Added docs for `requires` and for `system_info('server_mods')`

* Better docs for requires (mention * and available prefixes)

* Update docs/scarpet/api/Overview.md

* Update docs/scarpet/api/Overview.md

* Update docs/scarpet/api/Overview.md

Co-authored-by: gnembon <41132274+gnembon@users.noreply.github.com>

docs/scarpet/api/Auxiliary.md

@@ -685,6 +685,7 @@ Available options in the scarpet app space:
  * `server_banned_players` - list of banned player names
  * `server_banned_ips` - list of banned IP addresses
  * `server_dev_environment` - boolean indicating whether this server is in a development environment.
+ * `server_mods` - map with all loaded mods mapped to their versions as strings
  
  System related properties
  * `java_max_memory` - maximum allowed memory accessible by JVM