Smart Linking: Implement "Apply to selection" mode

Is your feature request related to a problem?

Currently, our Smart Linking feature checks the selected block or whole post and determines the location where links should be inserted. This is fine, but to quote @jbao:

PCI users have often complained about the lack of flexibility in selecting the anchor word length, sometimes they already have some specific word/phrase in mind that they want to add link to, or they’re not happy with the link suggestion results and want to extend/shrink the suggested anchor words. In all these cases, we can provide users with more flexibility by adding Smart Linking to the toolbar, so that they can pre-select the best anchor words of their choice, click on the smart link icon. Our API will then suggest links based on the relevance to the anchor words, and return them in a dropdown menu.

Describe the solution you'd like

Add a Smart Linking icon to the toolbar, which will allow Smart Linking to run for the currently selected text. We still need to determine how the whole flow/UX should be when we start working on this.

Additional context

@jbao, wanted to check with you whether we'd be ready from an API standpoint regarding this, or if API work would be needed to make this possible.

No files, tests, or entry points are identified in the issue. Start by clarifying the toolbar flow for pre-selected text and whether the existing API supports anchor-word suggestions; done means Smart Linking can be invoked from the toolbar for selected text and presents relevant links in a dropdown.