Resolves #1576

Add missing type declarations for the properties of Exception class.

What has changed?

• Converted function to a class so as to declare fields.
• Declare all the properties (with the types inferred) that have been attached to the this object in the source file.
• Declare a missing second argument in the typing - node of type hbs.AST.Node.
• Add tests for the exception typing.

• Please don't start pull requests for security issues. Instead, file a report at https://www.npmjs.com/advisories/report?package=handlebars
• Maintain the existing code style
• Are focused on a single change (i.e. avoid large refactoring or style adjustments in untouched code if not the primary goal of the pull request)
• Have good commit messages
• Have tests
• Have the typings (lib/handlebars.d.ts) updated on every API change. If you need help, updating those, please mention that in the PR description.
• Don't significantly decrease the current code coverage (see coverage/lcov-report/index.html)
• Currently, the 4.x-branch contains the latest version. Please target that branch in the PR.

@nknapp Not sure if this change requires a test too. If yes, please suggest and show me some examples and I can do it. Thanks!

PS: I tried npm checkTypes and npm test which all passed for me - not sure what else I need to do.