move a11y strings into main strings json

[zepumph]

From phetsims/chipper#795. I think that @jessegreenberg should do this, because likely there should be some organizational refactoring done in this repo, in part because there are so many strings here.

[jbphet]

This issue was reviewed during today's sound design meeting, and it looks like the a11y strings are still separate from the other strings (they are in BASEA11yStrings.js). This doesn't seem like a blocking issue for the upcoming publication of version 1.5.0, but we thought it best to check. @jessegreenberg - Should we try to get this done now, or leave it for the next publication from master?

[zepumph]

I'll put in a strong preference to doing this sooner rather than later, and not keeping this technical debt lingering.

[jessegreenberg]

Agreed this should be done! But this also doesn't need to block taking RC SHAs. I won't be able to get to this until 9/7/21 at the earliest, @jbphet if you would like to take RC SHAs before then I think that is fine.