Add constants to outline view

[vitallium]

Description

Adds constants assignments to the symbols outline.
According to the Ruby specification almost everything is a constant:

class ApplicationController # ApplicationController is a constant
Bar = :bar # Bar is a constant
BAR = 1 # BAR is a constant
Bar = Class.new # Bar is a constant

The challenge is to show only the most necessary symbols in the symbols outline. Pulling all constants results in an output where class declarations are shown as well, and this is not what we want. The solution is to pull only assigned constants. We are fortunate here because, otherwise, the Ruby interpreter would produce an uninitialized constant error. Consider the following code:

# frozen_string_literal: true

ABC = 1
Goo = 2
Foo = 3

foo = 1

class Category < ApplicationRecord
  validates :name, presence: true, length: { maximum: 255 }, uniqueness: true
  validates :hidden, inclusion: { in: [ true, false ] }
end

class RubyBlog
  URL    = "rubyguides.com"
  AUTHOR = "Jesus Castello"
  SomeError = "Jesus Castello"
end

Opening the symbols outline panel produces the following:

Closes #50

[andyw8]

🤔 Why match on digits and underscores here? They're valid inside the constant name, but not at the beginning.

[vitallium]

That was a bold copy-and-paste from highlights.scm, I shouldn't have done it that way. I also found another case where this query matches the incorrect node in an assignment::

Category = Kernel

Such code produces two constants in the outline panel:

I think we should target places where the code assigns values to constants only:

(assignment left: (constant) @name) @item