Update builtin keywords

* Remove old and unsupported entries
* Add `@offsetOf` (previously known as `@byteOffsetOf`)
* Fix `@addWithOverflow` being split into `@add` and `@withOverflow`

Author: LemonBoy

syntax/zig.vim

@@ -90,8 +90,7 @@ let s:zig_syntax_keywords = {
     \ ,                "callconv"
     \ ,                "noalias"]
     \ , 'zigBuiltinFn': ["align"
-    \ ,                  "@add"
-    \ ,                  "@WithOverflow"
+    \ ,                  "@addWithOverflow"
     \ ,                  "@as"
     \ ,                  "@atomicLoad"
     \ ,                  "@atomicStore"
@@ -103,7 +102,6 @@ let s:zig_syntax_keywords = {
     \ ,                  "@cImport"
     \ ,                  "@cInclude"
     \ ,                  "@cUndef"
-    \ ,                  "@canImplicitCast"
     \ ,                  "@clz"
     \ ,                  "@cmpxchgWeak"
     \ ,                  "@cmpxchgStrong"
@@ -138,6 +136,7 @@ let s:zig_syntax_keywords = {
     \ ,                  "@src"
     \ ,                  "@bitOffsetOf"
     \ ,                  "@byteOffsetOf"
+    \ ,                  "@offsetOf"
     \ ,                  "@OpaqueType"
     \ ,                  "@panic"
     \ ,                  "@ptrCast"
@@ -151,8 +150,6 @@ let s:zig_syntax_keywords = {
     \ ,                  "@setRuntimeSafety"
     \ ,                  "@setEvalBranchQuota"
     \ ,                  "@setFloatMode"
-    \ ,                  "@setGlobalLinkage"
-    \ ,                  "@setGlobalSection"
     \ ,                  "@shlExact"
     \ ,                  "@This"
     \ ,                  "@hasDecl"
@@ -175,8 +172,6 @@ let s:zig_syntax_keywords = {
     \ ,                  "@typeName"
     \ ,                  "@TypeOf"
     \ ,                  "@atomicRmw"
-    \ ,                  "@bytesToSlice"
-    \ ,                  "@sliceToBytes"
     \ ,                  "@intToError"
     \ ,                  "@errorToInt"
     \ ,                  "@intToEnum"