fix(parse5-utils): add support for templates in `findNode`

[WillsonSmith]

What I did

1. Add a shared helper function for findNode and findNodes to find child nodes within <template> elements
2. Enable findNode to find children within a <template> element
3. Add a test for findNode

This expands on #2398 which added this functionality to findNodes.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 87afd9af92d53a45bee74c28f4bf4c93383bbce3

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@web/parse5-utils	Patch

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[WillsonSmith]

Always happy to contribute, thanks for taking the time to review this! 😄