Alt click + terminal button to split

[meganrogge]

This PR fixes #122117

[meganrogge]

Ideally would like to get the split icon to appear on hover + alt like the editor button does. looks like it would take a fair amount of duplication, since MenuIttemAction does this already, but can't be used or referenced where dropdownWithPrimaryActionViewItem lives.

Thus, I've added an altAction property to dropdownWithPrimaryActionViewItem.

[meganrogge]

Currently, an issue with this is that clicking + w alt depressed results in:

• a split terminal is created 👍🏼 bc onclick & altKey are monitored in dropdownWithPrimaryActionViewItem
• a non-split terminal is created 👎🏼 bc onclick in the parent class, BaseActionViewItem, runs the primaryAction on click.

Stuff I tried:

• to override the parent onClick method - problems w this include EventLike doesn't have an altKey I can check ❌
• stopping the event from propagating in the onClick within dropdownWithPrimaryActionViewItem - did not work ❌

All of this could be avoided if event: unknown in terminalActions was ever defined. ☹️