Add WeakRef (pangloss#1238)

Add syntax highlightling to WeakRef as defined in tc39/proposal-weakrefs

Author: João Vieira

syntax/javascript.vim

@@ -110,7 +110,7 @@ syntax keyword jsAsyncKeyword           async await
 syntax match   jsSwitchColon   contained /::\@!/        skipwhite skipempty nextgroup=jsSwitchBlock
 
 " Keywords
-syntax keyword jsGlobalObjects     ArrayBuffer Array BigInt64Array BigUint64Array Float32Array Float64Array Int16Array Int32Array Int8Array Uint16Array Uint32Array Uint8Array Uint8ClampedArray Boolean Buffer Collator DataView Date DateTimeFormat Function Intl Iterator JSON Map Set WeakMap WeakSet Math Number NumberFormat Object ParallelArray Promise Proxy Reflect RegExp String Symbol Uint8ClampedArray WebAssembly console document fetch window
+syntax keyword jsGlobalObjects     ArrayBuffer Array BigInt64Array BigUint64Array Float32Array Float64Array Int16Array Int32Array Int8Array Uint16Array Uint32Array Uint8Array Uint8ClampedArray Boolean Buffer Collator DataView Date DateTimeFormat Function Intl Iterator JSON Map Set WeakMap WeakRef WeakSet Math Number NumberFormat Object ParallelArray Promise Proxy Reflect RegExp String Symbol Uint8ClampedArray WebAssembly console document fetch window
 syntax keyword jsGlobalNodeObjects  module exports global process __dirname __filename
 syntax match   jsGlobalNodeObjects  /\<require\>/ containedin=jsFuncCall
 syntax keyword jsExceptions         Error EvalError InternalError RangeError ReferenceError StopIteration SyntaxError TypeError URIError