Bug 1988206 - Record clicks on elements in Shadow DOM

[LeoMcA]

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1988206
Relates to mdn/fred#752

Uses composedPath() to get the original target within a Shadow DOM, rather than the custom element with a Shadow DOM itself (which is what event.target is).

This should possibly be considered a breaking change: if clients have patched this themselves, by firing GleanMetrics.recordElementClick(); for relevant clicks in Shadow DOMs (as we've done in fred), this change upstream would result in duplicated pings.

Tests

I've tested with a local build of glean, I couldn't find any tests with related to this data-glean-id auto pinging: I'm happy to add a test if pointed in the right direction.

Pull Request checklist

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