Add Provider API

[joefitzgerald]

🚨 WIP - Do Not Merge 🚨

Description of the Change

This PR seeks to implement the proposal described at https://github.com/atom/symbols-view/wiki/v1-Proposal.

Benefits

• External packages will be able to extend the functionality included in symbols-view, by providing grammar-specific strategies for navigation to symbols definitions or references
• Users will have new functionality allowing them to view references to a symbol and navigate to those references
• ctags support in symbols-view will be encapsulated in a provider, allowing for further enhancement of symbols-view in ways that might go beyond what ctags can allow

Possible Drawbacks

Many packages may be directly integrating with symbols-view by activating the package and attempting to use functions that are not part of a public API. Such packages will need to be updated to continue to function correctly.

Applicable Issues

• WIP: Add Symbol Provider API #95
• Add service-hub Based API For Symbols Providers #70
• Add GoDef Support joefitzgerald/go-plus#123
• find definition? joefitzgerald/go-plus#33
• Add 'Go To Definition' Support joefitzgerald/go-plus#11

Tasks

• Add symbol provider API
• Implement Goto Definition Request
• Implement Find References Request
• Implement Document Symbols Request
• Extract CtagsProvider and register it like any other provider
• Rewrite go-to-view.js to use etch
• Rewrite go-back-view.js to use etch
• Add find references to provider API
• Add find references view
• Add find references commands, keymaps, menu(s)

/cc @zmb3

🚨 WIP - Do Not Merge 🚨

[tillkruss]

Any updates on this?