Add `scope` attribute for table block headers

[pattonwebz]

As part of a project I am working on with Equalize Digital I needed to add a scope value to some tables to meet a11y best practices.

Relevant WCAG SC: 1.3.1 - Info and Relationships (Level A)

I want to be able to set scope="col" on the header when it's the row. And scope="row" on them when it is set to column header.

[pattonwebz]

@amberhinds Opened this issue and made a PR for it to add scopes for the table block.