* Added FSE support to Alves.
* Note: the theme `#masthead` style were preventing the FSE header to be full width as intended. Instead of needlessly overriding the whole `header.php`, I've opted to add a simpler `body:not( .fse-enabled ) #masthead`.
* Update Varia (and the other FSE themes) with FSE improvements:
* Hide the (+) icons when rendering the Template Part block inside the Page editor (especially useful for the Columns block).
* Remove the top margin if the Site Title block is the first child of an editor layout.
* Make sure each column in a Columns block is max width, so that nested FSE blocks can behave as expected.
