ci: Add license header check for pull requests

Co-Authored-By: OpenSauce <48618519+opensauce04@users.noreply.github.com>

.ci/license-header.rb

@@ -0,0 +1,47 @@
+#!/usr/bin/ruby
+# frozen_string_literal: true
+
+license_header = <<~EOF
+      // SPDX-FileCopyrightText: Copyright yuzu/Citra Emulator Project / Eden Emulator Project
+      // SPDX-License-Identifier: GPL-3.0-or-later
+      EOF
+
+print 'Getting branch changes...'
+branch_name = `git rev-parse --abbrev-ref HEAD`.chomp
+branch_commits = `git log #{branch_name} --not master --pretty=format:"%h"`.split("\n")
+branch_commit_range = "#{branch_commits[-1]}^..#{branch_commits[0]}"
+branch_changed_files = `git diff-tree --no-commit-id --name-only #{branch_commit_range} -r`.split("\n")
+puts 'done'
+
+print 'Checking files...'
+issue_files = []
+branch_changed_files.each do |file_name|
+   if file_name.end_with?('.cpp', '.h', '.kt', '.kts') and File.file?(file_name)
+      file_content = File.read(file_name)
+      if not file_content.start_with?(license_header)
+         issue_files.push(file_name)
+      end
+   end
+end
+puts 'done'
+
+if issue_files.empty?
+   puts "\nAll changed files have correct headers"
+   exit 0
+end
+
+puts <<-EOF
+
+The following #{issue_files.length} files have incorrect license headers:
+#{issue_files.join("\n")}
+
+The following license header should be added to the start of all offending files:
+=== BEGIN ===
+#{license_header}
+===  END  ===
+
+If some of the code in this PR is not being contributed by the original author, the files which have been exclusively changed by that code can be ignored.
+If this happens, this PR requirement can be bypassed once all other files are addressed.
+EOF
+
+exit 1