Parse `function (:*=(f))() end` syntax compatibly

[c42f]

Slightly modify the anonymous function arg list disambiguation rules so that the function signature in this syntax parses as

(call (call (quote *=) f))

The package NiLang uses this, so we'd like to keep it working.

Fixes the final niggling case in #134.

See also https://github.com/GiggleLiu/NiLang.jl/issues/83

[codecov[bot]]

Codecov Report

Merging #309 (a562455) into main (8a47045) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #309   +/-   ##
=======================================
  Coverage   96.81%   96.81%           
=======================================
  Files          14       14           
  Lines        4107     4109    +2     
=======================================
+ Hits         3976     3978    +2     
  Misses        131      131           

Impacted Files	Coverage Δ
src/parser.jl	98.33% <100.00%> (+<0.01%)	:arrow_up: