fix GH-272: Highlight arrow funcs without parenthesized params

Arrow expressions with "naked" single parameters are currently not being
recognized as such, due to the mis-labeling of the expression as assignment.

```js
// The part: `x =` looks like assignment to syntax rules:
let myFunc = x => 3;
```

This change introduces a negative lookahead for the `>` character in
the assignment matcher.

Author: Chris Carpita

syntax/javascript.vim

@@ -108,7 +108,7 @@ syntax match   jsFloat            /\<-\=\%(\d\+\.\d\+\|\d\+\.\|\.\d\+\)\%([eE][+
 syntax match   jsObjectKey        /\<[a-zA-Z_$][0-9a-zA-Z_$]*\>\(\s*:\)\@=/ contains=jsFunctionKey contained
 syntax match   jsFunctionKey      /\<[a-zA-Z_$][0-9a-zA-Z_$]*\>\(\s*:\s*function\s*\)\@=/ contained
 
-syntax match   jsAssignmentExpr     /\v%([a-zA-Z_$]\k*\.)*[a-zA-Z_$]\k*\s*\=/ contains=jsFuncAssignExpr,jsAssignExpIdent,jsPrototype,jsOperator,jsThis,jsNoise,jsArgsObj
+syntax match   jsAssignmentExpr     /\v%([a-zA-Z_$]\k*\.)*[a-zA-Z_$]\k*\s*\=\(>\)\@!/ contains=jsFuncAssignExpr,jsAssignExpIdent,jsPrototype,jsOperator,jsThis,jsNoise,jsArgsObj
 syntax match   jsAssignExpIdent     /\v[a-zA-Z_$]\k*\ze%(\s*\=)/ contained
 syntax match   jsFuncAssignExpr     /\v%(%([a-zA-Z_$]\k*\.)*[a-zA-Z_$]\k*\s*\=\s*){-1,}\ze%(function\s*\*?\s*\()/ contains=jsFuncAssignObjChain,jsFuncAssignIdent,jsFunction,jsPrototype,jsOperator,jsThis,jsArgsObj contained
 syntax match   jsFuncAssignObjChain /\v%([a-zA-Z_$]\k*\.)+/ contains=jsPrototype,jsNoise contained