Allow importing library as ESM

[mohd-akram]

This allows importing the library directly from an ESM, without requiring a build step. One change was required for a test to pass in index.runtime.js - a copy of the runtime module object is made to allow overriding checkRevision (ESMs don't allow modifying import objects). The source-map module is replaced via subpath imports rather than via runtime checking since require is not available in a ESM.

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[jaylinski]

Nice work, I had this on my backlog too. But we won't merge this in 4.x in order to keep it backwards-compatible. Please target the PR at the master-branch.

[mohd-akram]

Yes, I hoped it could be done in 4.x but reached the same conclusion. Will be retargeting and adjusting the PR - it needs some changes most likely - but need to merge the CI and build PRs first.

[mohd-akram]

Rebased on master, but closing for now as it depends on updating the build system, as well as fixing @handlebars/parser's exports (it does not properly provide ESM support as it is missing an exports field in its package.json) for integration tests to pass.