They should by default scope over the group in which they
are defined (except `\gdef` and `\xdef`, which are global).
In addition, environments must be treated as groups.
We handle this by making sMacros in the LaTeX parser state
a STACK of macro tables. Opening a group adds a table to
the stack, closing one removes one. Only the top of the stack
is queried.
This commit adds a parameter for scope to the Macro constructor
(not exported).
Closes#7494.
