[ruby/prism] [Bug #21010] Reject endless method definition of []=

Fixes: https://bugs.ruby-lang.org/issues/20785

ruby/prism@192960ce5d

Author: ydah

test/prism/errors/defs_endless_method.txt

@@ -0,0 +1,12 @@
+def f=(k,v)=1
+    ^~ invalid method name; a setter method cannot be defined in an endless method definition
+
+def obj.f=(k,v)=1
+        ^~ invalid method name; a setter method cannot be defined in an endless method definition
+
+def []=(k,v)=1
+    ^~~ invalid method name; a setter method cannot be defined in an endless method definition
+
+def obj.[]=(k,v)=1
+        ^~~ invalid method name; a setter method cannot be defined in an endless method definition
+