Fix ty var parsing on class head (purescript#4523)

* Fix ty var parsing on class head

Author: JordanMartinez

CHANGELOG.d/fix_fix-class-head-ty-var.md

@@ -0,0 +1 @@
+* Fix parsing bug where `@var` was allowed in type class head

src/Language/PureScript/CST/Parser.y

@@ -722,7 +722,7 @@ classSuper :: { (OneOrDelimited (Constraint ()), SourceToken) }
   : constraints '<=' {%^ revert $ pure ($1, $2) }
 
 classNameAndFundeps :: { (Name (N.ProperName 'N.ClassName), [TypeVarBinding ()], Maybe (SourceToken, Separated ClassFundep)) }
-  : properName manyOrEmpty(typeVarBinding) fundeps {%^ revert $ pure (getProperName $1, $2, $3) }
+  : properName manyOrEmpty(typeVarBindingPlain) fundeps {%^ revert $ pure (getProperName $1, $2, $3) }
 
 fundeps :: { Maybe (SourceToken, Separated ClassFundep) }
   : {- empty -} { Nothing }

tests/purs/failing/4522.out

@@ -0,0 +1,10 @@
+Error found:
+at tests/purs/failing/4522.purs:4:11 - 4:12 (line 4, column 11 - line 4, column 12)
+
+  Unable to parse module:
+  Unexpected token '@'
+
+
+See https://github.com/purescript/documentation/blob/master/errors/ErrorParsingModule.md for more information,
+or to contribute content related to this error.
+

tests/purs/failing/4522.purs

@@ -0,0 +1,4 @@
+-- @shouldFailWith ErrorParsingModule
+module Main where
+
+class Foo @a