Expand `except:` to have a replace gesture

[mtoy-googly-moogly]

exceptClause
  : id
  | id IS expression
  ;

This would enable the following desired gestures ...

run: x -> {
  select: * { except: name_in_output is name_in_input }
}

source: xsrc is ysrc extend {
  except: thing is thing::number // stored as a string in ysrc, usable as a number here
}

• [ ] @lloydtabb
• [ ] @carlineng
• [x] @christopherswenson
• [x] @mtoy-googly-moogly

[mtoy-googly-moogly]

This would fix https://github.com/malloydata/malloy/issues/1162