Add nullish coalescing operator (??)

Author: Andrei Bartashevich

syntax/typescript.vim

@@ -206,7 +206,7 @@ syntax keyword typescriptFuncKeyword function
 syn match typescriptBraces "[{}\[\]]"
 syn match typescriptParens "[()]"
 syn match typescriptEndColons "[;,]"
-syn match typescriptLogicSymbols "\(&&\)\|\(||\)\|\(!\)"
+syn match typescriptLogicSymbols "\(&&\)\|\(||\)\|\(??\)\|\(!\)"
 syn match typescriptOpSymbols "=\{1,3}\|!==\|!=\|<\|>\|>=\|<=\|++\|+=\|--\|-="
 
 " typescriptFold Function {{{