gh-123562: Improve SyntaxError message for case ... as a.b

[sobolevn]

Notice that there's still a case that is not reported very well, but it is a rather hard one:

>>> match 1:
...     case x as a + 1: ...
...     
  File "<python-input-0>", line 2
    case x as a + 1: ...
                ^
SyntaxError: invalid syntax

This happens because pattern_capture_target actually matches case x as a but not + 1

• Issue: Improve SyntaxError message for case ... as a.b #123562

[cfbolz]

Looks good to me!

[lysnikolaou]

LGTM! Thanks @sobolevn!