Disallow trailing junk in generators

[c42f]

Disallow generator syntax with trailing non-delimited expressions

(y for x in xs a)
(y for x in xs a, b)

Allow parameter syntax such as

f(y for x in xs; a)

because the flisp parser allowed this it's used in Base in at least one location.

Disallow the almost-equivalent block syntax for now:

(y for x in xs; a)

because the flisp parser disallowed it and users can always get the same thing with some extra parentheses.

Fixes #407

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.18%. Comparing base (6532515) to head (a74fb9c).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #497      +/-   ##
==========================================
- Coverage   96.21%   96.18%   -0.04%     
==========================================
  Files          13       13              
  Lines        4171     4033     -138     
==========================================
- Hits         4013     3879     -134     
+ Misses        158      154       -4     

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