[Snyk] Security upgrade handlebars from 4.1.2 to 4.6.0

[snyk-bot]

Snyk has created this PR to fix one or more vulnerable packages in the `npm` dependencies of this project.

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Changes included in this PR

• Changes to the following files to upgrade the vulnerable dependencies to a fixed version:
  • metalsmith/package.json
  • metalsmith/package-lock.json

Vulnerabilities that will be fixed

With an upgrade:

Severity	Issue	Breaking Change	Exploit Maturity
	Prototype Pollution SNYK-JS-HANDLEBARS-567742	No	Proof of Concept

Commit messages

Package name: handlebars

The new version differs by 112 commits.

• 91a1b5d v4.6.0
• 770d746 Update release notes
• d7f0dcf refactor: fix typo in private test method
• 187d611 test: add path to nodeJs when running test:bin
• d337f40 test: show diff when test:bin fails
• d03b6ec feat: access control to prototype properties via whitelist
• 164b7ff chore: ignore .nyc_output
• ac4655e chore: disable "dot-notation" rule
• 14b621c test/style: remove or hide unused code in git.js, add tests
• 1ec1737 test/style: refactor remaining grunt tasks to use promises instead of callbacks
• 1ebce2b test/style: use nyc instead of istanbul, npm audit fix
• 3a5b65e test/style: refactor parser task
• dde108e test/style: refactor test-task to make it more readable
• dc54952 chore: change eslint-rules for tasks/
• d1fb07b Update (C) year in the LICENSE file
• 04b1984 chore: try to fix saucelabs credentials (#1627)
• c40d9f3 chore: active linting and formatting on commit
• 8901c28 chore: fix task name in build
• e97685e style: reformat all files using prettier
• e913dc5 chore: restructure build commands
• 1f61f21 chore: configure prettier and eslint
• 587e7a3 remove yarn.lock
• edcc84f Update readme.md with updated links (#1620)
• 23d58e7 fix(runtime.js): partials compile not caching (#1600)

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