refactor: configs are now fully resolved ahead of time in one pass

This turned out to be a pretty big refactor. The key changes made are:
- The context for template strings is much more clearly defined
  and documented.
- The context is now the same for every module and is fully prepared
  on startup instead of needing multiple passes and having different
  keys depending on when/where it is used. This does mean that some
  existing template strings needed to be changed, but they were not
  a properly supported feature anyway.
- Some functionality has now been moved out of the PluginContext and
  back to the Garden class. In turn, Command actions are now passed a
  Garden instance.
- Module and Service are now interfaces instead of classes, so they are
  now simple objects that are passed around. This cleans up the API for
  plugins, and also paves the way for out-of-process plugins.
- ServiceConfigs and TestConfigs are now attached directly to
  ModuleConfig objects.

docs/reference/README.md

@@ -2,3 +2,4 @@
 
 * [Commands](./commands.md)
 * [Config](./config.md)
+* [Template strings](./template-strings.md)