Make current context available as placeholder in field options #166

Summary

This issue only concerns multi-context-mode. This is more of an enhancement than a bug. In some fields it's possible to make snippet calls so tags get processed and you can for example create your own set up input options by calling pdoResources to populate them. This is not context-aware.

Step to reproduce

Try to find a context placeholder and fail at it.

Observed behavior

Me not finding the context placeholder :)

Expected behavior

Me having a [[+clientconfig.context]] placeholder of some sort, so I can do this:

[[pdoResources? 
   &tpl=`@INLINE [[+pagetitle]]==[[+id]]`
   &parents=`0`
   &limit=`0`
   &context=`[[+clientconfig.context:default=`web`]]`
   &outputSeparator=`||`
   &showHidden=`1`
]]

Environment

ClientConfig 2.1.0, MODX 2.7.1, Debian, MySQL 5.7.25, PHP 7.0.33