Add linter to find unused helper methods

This script attempts to detect unused helper methods.

scripts/lint/unused_helper_methods.rb

+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+# Inspired in part by https://gist.github.com/kylefox/617b0bead5f53dc53a224a8651328c92
+#
+
+require 'parallel'
+
+start = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+
+# Build an array of filename globs to process.
+# Only search file types that might use or define a helper.
+#
+extensions = %w[rb haml erb].map { |ext| "{ee/,}app/**/*.#{ext}" }
+
+# Build a hash of all the source files to search.
+# Key is filename, value is an array of the lines.
+#
+source_files = {}
+
+Dir.glob(extensions).each do |filename|
+  source_files[filename] = File.readlines(filename)
+end
+
+# Build an array of {method, file} hashes defined in app/helper/* files.
+#
+helpers = source_files.keys.grep(%r{app/helpers}).flat_map do |filename|
+  source_files[filename].flat_map do |line|
+    line =~ /def ([^(\s]+)/ ? [{ method: Regexp.last_match(1).chomp, file: filename }] : []
+  end
+end
+
+puts "Scanning #{source_files.size} files for #{helpers.size} helpers..."
+
+# Combine all the source code into one big string, because regex are fast.
+#
+source_code = source_files.values.flatten.join
+
+# Iterate over all the helpers and reject any that appear anywhere in the complete source.
+#
+unused = Parallel.flat_map(helpers, progress: 'Checking helpers') do |helper|
+  /(?<!def )#{Regexp.quote(helper[:method].sub(/^self\./, ''))}\W/.match?(source_code) ? [] : helper
+end
+
+finish = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+
+if unused
+  puts "\nFound #{unused.size} unused helpers:\n\n"
+  unused.each { |helper| puts "  - [ ] `#{helper[:file]}`: `#{helper[:method]}`" }
+  puts "\n"
+else
+  puts 'No unused helpers were found.'
+end
+
+puts "Finished in #{finish - start} seconds."