update src/core/index.js to export all global APIs, deprecate old globals…

[trusktr]

...in favor of a single global DOCSIFY, and add tests for this

@anikethsaha This updates #1192 to fix the issues I found there.

The build now works, and the change is backwards compatible: it leaves the old globals in place but additionally creates a new single global variable DOCSIFY that we should recommend in favor of the other multiple globals which we should deprecate in the docs.

I don't think this needs to be in a major version bump as people can still use the old globals, unaffected (as per the new tests in this PR). Then in the next major we can remove the old global. Plus we can add an .esm.js bundle and recommend for people to use that instead of the global version so that they don't pollute the global space.

This new DOCSIFY global has the Docsify constructor exposed, so we can start heading in a direction where we prefer people to do new DOCSIFY.Docsify(options) instead of window.$docsify = options. In a following PR, I can add the new behavior but keep the old $docsify options working for backwards compatibility without having to save it for a major bump.

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✅ Preview: https://docsify-previe-git-deprecate-old-globals-and-make-new-si-32a16c.docsify-core.now.sh

[trusktr]

Semantic Pull Request — add a semantic commit or PR title

Ah, nice check! Let me fix that!

[anikethsaha]

@trusktr can you submit a PR with updated docs where-ever needed.