add @constCast and @volatileCast

Author: andrewrk

syntax/zig.vim

@@ -108,6 +108,7 @@ let s:zig_syntax_keywords = {
     \ ,                  "@cmpxchgStrong"
     \ ,                  "@compileError"
     \ ,                  "@compileLog"
+    \ ,                  "@constCast"
     \ ,                  "@ctz"
     \ ,                  "@popCount"
     \ ,                  "@divExact"
@@ -189,6 +190,7 @@ let s:zig_syntax_keywords = {
     \ ,                  "@frameSize"
     \ ,                  "@bitReverse"
     \ ,                  "@Vector"
+    \ ,                  "@volatileCast"
     \ ,                  "@sin"
     \ ,                  "@cos"
     \ ,                  "@tan"