Add Type Definition for Handlebars.VERSION, Fixes #1647

[papasmile]

Addresses #1647

ran npm run check-before-pull-request locally with success.

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Before creating a pull-request, please check https://github.com/wycats/handlebars.js/blob/master/CONTRIBUTING.md first.

Generally we like to see pull requests that

• 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]

Thanks