Decorate production components with numbers & provide hover support over $n/$$ references in actions

[rail5]

This change provides editor decorations for each component of a production, showing $1:, $2:, etc

It also provides hover information over $n/$$ references inside the action body, displaying which production component is being referred to by this.

In order to accomplish this change:

• The parse function now stores the separator bars in its components list so that these can be used to distinguish alternative productions for correct component numbering
• A getParsedDocument method was added to the LanguageMode interface, so the already-parsed YaccDocument could be returned for use by the decoration function