ctags: enable basic support for Groovy files
Created by: creachadair
Add a language definition and regex rules based on the vim-gradle plugin at https://bitbucket.org/sw-samuraj/vim-gradle. The rules from the plugin have been modified as follows:
- Remove duplicate tag usage and fix labels not valid in universal-ctags.
- Remove prefixes and type tag decorations on method/constructor names.
- Consolidate rules for different visibilities.
- Fix some overlapping matches on different kinds.
- Fix incorrect regexp usage