limel-markdown: Add unit tests for markdownParser

[adrianschmidt]

At the moment, we can't write tests for the markdownParser because it uses a package called unified, which is written in ESM (ES Modules), with no non-ESM build included in the package. We're using an old version of Jest, which can't handle ESM, and is so old that the experimental option for allowing ESM doesn't exist in it. Newer Jest versions have it.

One option is to configure Babel to transpile ESM to a non-ESM version, but a) I've tried that and couldn't get it to work without breaking loads of other things, and b) then we're technically not actually testing the code that we're running in the browser. (I guess that's already true for packages that have non-ESM builds included.)

The preferable option is to update Jest to a version that can handle ESM. We are currently using Stencil 3, which locks jest and jest-cli to major version 27. (The latest major version of each is 29.) So we need to upgrade to Stencil 4 in order to upgrade to a newer version of Jest.

There is already a branch for updating to Stencil 4, and that works just fine, except that Stencil 4 requires an update of Typescript from v4 to v5, which conflicts with the current version of Kompendium, so Kompendium needs to be updated to work with Typescript v5.

Stuff that seems to be necessary to do before we can test markdownParser

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1. Make Jest able to handle ESM-only modules (like unified)
2. Upgrade to Stencil 4 #3207
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3. Deps update jgroth/kompendium#149
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