Improve how some Expr are shown by show_unquoted in base/show.jl.

Fixe some failing doctests in doc/src/mamual/metaprogramming.md.
Some other small tweaks following Jeff Bezanson suggestions
(see #32423).

Fixe a bug which caused errors when trying to show stuff like
"Base.@int128_str \"11111111111111111111\"".

Restrict the class of "special syntax" macrocalls which
benefit of the modifications this PR is about, according to
discussion at #32423

Change how nested interpolations are displayed (omitting unrequired parenthesis).

Add test for nested quotes and interpolations.

Correct small bug preventing correct display of "t[a b;]".

Update metaprogramming docs to better reflect changes in how quoting and unquoting get displayed.

Implement quote level counting mechanism to handle displaying
of Expr "with more unquotes than quotes".

Adde more tests for quote_level mechanism.

Include unhandled case into quote_level mechanism.

Rebase and squash previous commits.
Fix conflicts due to introduction of var"#funky name" syntax.

Add a couple of tests for exotic heads mixed with nested quoting and
interpolation.

Handle repr of macrocall expressions with
qualified macroname argument.