adjust check syntax so it highlights all identifiers with the same
binding site (with a green/yellow bubble) when the mouse moves over
any one of them

This change comes about because of the recent fixes to the
interactivity wrt to the rename menu. Basically, in order to fix the
bug (but still preserve the interactivity optimization), check syntax
changed from sending the information "here is a place to offer a
rename for these identifiers" to "here is a set of identifiers that
are all free-identifier=?" (the difference being that the latter does
not imply you got them all (which enables the optimization) and that
the information is slightly less rename-menu specific (which enables
the change in this commit))
Rackety
make the online check syntax error message font
size match the user's preferred font size

(useful in a classroom / talk setting)
fix close paren correction code
Fix typo.
More: clarify how to use DrRacket instead of plain `racket'