Add GtkAccessibleText interface for text widgets

Now that GtkAccessible is public, we're going to need a public interface for widgets that provide textual data.

Implementation details:

• AT-SPI: https://gnome.pages.gitlab.gnome.org/at-spi2-core/devel-docs/doc-org.a11y.atspi.Text.html
• UIAutomation: https://learn.microsoft.com/en-us/dotnet/api/system.windows.automation.provider.itextprovider?view=windowsdesktop-7.0
• NSAccessibilityProtocol: https://developer.apple.com/documentation/appkit/nsaccessibilityprotocol#1674786

See also: vte#2614