feat: Add new forceActionsMenu embed option

[vdonato]

This PR adds a new forceActionsMenu embed option that can be used to force vega-embed
to display actions in a details menu rather than in links even when defaultStyle is set to false.
If defaultStyle is not false or if actions is false, the option has no effect.

This is useful in cases where the parent application of an embedded vega chart wants to be able to
style the actions menu itself rather than having vega inline a <style/> tag to do so (see #1214 for
details as to why this might be desired). Currently, applications that want to do this have to use a small
hack of setting defaultStyles to an empty string rather than false so that vega still adds the menu
(see this code in the streamlit codebase for an example). This change doesn't really change anything
functionally but allows applications to write code that's a bit more clear.

[vdonato]

Hey @domoritz, heads up that I made this small change to fix a small issue that I came across in a change to the Streamlit OS lib. We're currently addressing this using a workaround, but it would be nice to have a less hacky solution (see the link in the PR description). Let me know if you have any other ideas for equivalent ways of improving this.

[domoritz]

Thank you for the feature