Adding support for nullish operator (pangloss#1165)

Author: amadeus

syntax/javascript.vim

@@ -156,7 +156,9 @@ syntax region  jsModuleGroup        contained matchgroup=jsModuleBraces        s
 syntax region  jsSpreadExpression   contained matchgroup=jsSpreadOperator      start=/\.\.\./ end=/[,}\]]\@=/ contains=@jsExpression
 syntax region  jsRestExpression     contained matchgroup=jsRestOperator        start=/\.\.\./ end=/[,)]\@=/
 syntax region  jsTernaryIf                    matchgroup=jsTernaryIfOperator   start=/?:\@!/  end=/\%(:\|}\@=\)/  contains=@jsExpression extend skipwhite skipempty nextgroup=@jsExpression
-syntax match   jsOperator       /?\.\ze\_D/
+" These must occur here or they will be override by jsTernaryIf
+syntax match   jsOperator           /?\.\ze\_D/
+syntax match   jsOperator           /??/ skipwhite skipempty nextgroup=@jsExpression
 
 syntax match   jsGenerator            contained /\*/ skipwhite skipempty nextgroup=jsFuncName,jsFuncArgs,jsFlowFunctionGroup
 syntax match   jsFuncName             contained /\<\K\k*/ skipwhite skipempty nextgroup=jsFuncArgs,jsFlowFunctionGroup