tabs show the last option by default

[julienrf]

Tabs show the last option by default, which is undesirable when we write about Scala CLI, sbt and Mill:

Instead, we should display the first tab by default.

[bishabosha]

for now you can write defaultTab in one of the tabs

[bishabosha]

but I think the only place where last as default is most desired is the install tabs (already marked with defaultTab) - so this change can be made in only the plugin code :)