Fix go-to-declaration for Ruby identifiers in some cases

[mislav]

• Fixes jumping to foo?/foo! declarations when the current scope is an ERB template.
• New feature: enables selecting text such as Foo::Bar and jumping to that as a tag to look up. When there is no selection, falls back to word under cursor.

More details in commit messages.

[mislav]

Now with many more fixes:

• Fixes jumping to foo? method if the cursor is just after the ! in the expression if !foo?. Due to limitations of getCurrentWordBufferRange, now switched to scanInBufferRange.
• Allow looking up foo= tags as well. If the expression is foo = (with space), look up foo= tag instead and fall back to foo as before.
• With cursor over Foo inside A::Foo::Bar, first look up the fully-qualified constant A::Foo (this will resolve when the tags file was generated with --extra=q option) and fall back to Foo as before.

[thedaniel]

This looks good to me. @atom/feedback ?

[lee-dohm]

Looks good to me too :+1:

[mislav]

Thank you @BinaryMuse @thedaniel!