feat: add custom tag

[scrpgil]

This pull request is for DocSearch custom tags. This pull request does not change its behavior or appearance.

I am translating the stencil-site into Japanese(Site is stenciljs.jp). And I want to be able to search that site with DocSearch.

To do this, we will use the custom tag function of DocSearch. For details, please see this pull request(This pull request is Step 2) https://github.com/ionic-team/stencil-site/issues/723

If this pull request is passed, perform the work of Step 3 and install DocSearch in the Japanese document of StencilJS.

Thanks you!

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