WIP - Update d2l-tooltip to use PopoverMixin

[dbatiste]

GAUD-7355

This PR is a work in progress. It defines a new class for d2l-tooltip that extends PopoverMixin. Most of the opener behaviour is duplicated from the original, but I've taken the opportunity to convert the members to actually be private. PopoverMixin basically replaces all the original tooltip positioning logic.

Currently it simply displays the tooltip using PopoverMixin but there is remaining work:

• Adapting the complete d2l-tooltip API positioning options
• Applying style overrides to the popover so it looks the same as tooltips (ex. dark background)

The original code is unchanged with the exception of factoring out a couple helper methods.

I will add some comments to the PR to help review.

Flagged (off) with GAUD-7355-tooltip-popover.

[github-actions[bot]]

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[d2l-github-release-tokens[bot]]

:tada: This PR is included in version 3.144.0 :tada:

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