Expose CollapsibleTitle publicly

[dillydill123]

I want to query CollapsibleTitles and call parent.screen.focus_next(CollapsibleTitle). To do this, I need to do the following:

from textual.widgets._collapsible import CollapsibleTitle

This causes issues with linters that do not allow private imports (_collapsible here). Could this type CollapsibleTitle be exposed publicly? Perhaps in textual.types?

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[willmcgugan]

Have a look in the widgets directory to see how we do this for other widgets. But it would be easy to allow from textual.widgets.collapsible import CollapsibleTitle to work. A PR would be appreciated.

[dillydill123]

Here's my PR attempt