[8.6] [@kbn/handlebars] housekeeping (#146174) (#146293)

# Backport

This will backport the following commits from `main` to `8.6`:
- [[@kbn/handlebars] housekeeping
(#146174)](#146174)

<!--- Backport version: 8.9.7 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

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housekeeping (#146174)\n\nJust a few housekeeping commits to improve the
README and tests of the\r\n`@kbn/handlebars`
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housekeeping (#146174)\n\nJust a few housekeeping commits to improve the
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housekeeping (#146174)\n\nJust a few housekeeping commits to improve the
README and tests of the\r\n`@kbn/handlebars`
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Co-authored-by: Thomas Watson <watson@elastic.co>