Recognize more CloudFormation templates

[PatMyron]

Since AWSTemplateFormatVersion is optional, this CloudFormation template recognition logic won't recognize some CloudFormation templates:

vscode-cfn-nag/server/src/server.ts

Lines 186 to 191 in 2313625

	const is_cfn_regex = new RegExp('"?AWSTemplateFormatVersion"?');
	let is_cfn = false;
	const text = document.getText();
	if (is_cfn_regex.exec(text)) {
	is_cfn = true;
	}

Similar discussions: aws-cloudformation/cfn-lint-visual-studio-code#99, aws/aws-toolkit-jetbrains#1715 (comment), https://github.com/github/super-linter/pull/231#discussion_r445708695

The CloudFormation Linter Visual Studio Code extension has CloudFormation template recognition logic in TypeScript that can be borrowed