remove the zigVarDecl group which maps to Function

The previous mapping made no sense. The mapping of threadlocal and
comptime will be changed in future commits.

As a reference I used the builtin syntax highlighting from neovim for
the following languages that have similar keywords to Zig's `var` and
`const`:

- Rust: `let`
- Go: `var`
- Javascript: `var`,`let`, `const`

They all map their keywords to the Keyword syntax highlighting group.

Author: Techatrix

syntax/zig.vim

@@ -23,12 +23,11 @@ syntax keyword zigComparatorWord and or orelse
 syntax keyword zigStructure struct enum union error packed opaque
 syntax keyword zigException error
 syntax keyword zigStorageClass
-syntax keyword zigVarDecl var const comptime threadlocal
 syntax keyword zigDummyVariable _
-syntax keyword zigKeyword fn try test pub usingnamespace
+syntax keyword zigKeyword var const fn try test pub comptime usingnamespace
 syntax keyword zigExecution return break continue
 syntax keyword zigMacro defer errdefer async nosuspend await suspend resume export extern
-syntax keyword zigPreProc addrspace align asm callconv catch inline noalias noinline
+syntax keyword zigPreProc addrspace align asm callconv catch inline noalias noinline threadlocal
 syntax keyword zigBuiltinFn @addrSpaceCast @addWithOverflow @alignCast @alignOf @as @atomicLoad @atomicRmw @atomicStore @bitCast @bitOffsetOf @bitSizeOf @branchHint @breakpoint @mulAdd @byteSwap @bitReverse @offsetOf @call @cDefine @cImport @cInclude @clz @cmpxchgStrong @cmpxchgWeak @compileError @compileLog @constCast @ctz @cUndef @cVaArg @cVaCopy @cVaEnd @cVaStart @disableInstrumentation @disableIntrinsics @divExact @divFloor @divTrunc @embedFile @enumFromInt @errorFromInt @errorName @errorReturnTrace @errorCast @export @extern @field @fieldParentPtr @FieldType @floatCast @floatFromInt @frameAddress @hasDecl @hasField @import @inComptime @intCast @intFromBool @intFromEnum @intFromError @intFromFloat @intFromPtr @max @memcpy @memmove @memset @min @wasmMemorySize @wasmMemoryGrow @mod @mulWithOverflow @panic @popCount @prefetch @ptrCast @ptrFromInt @rem @returnAddress @select @setEvalBranchQuota @setFloatMode @setRuntimeSafety @shlExact @shlWithOverflow @shrExact @shuffle @sizeOf @splat @reduce @src @sqrt @sin @cos @tan @exp @exp2 @log @log2 @log10 @abs @floor @ceil @trunc @round @subWithOverflow @tagName @This @trap @truncate @Type @typeInfo @typeName @TypeOf @unionInit @Vector @volatileCast @workGroupId @workGroupSize @workItemId
 
 syntax match zigType "\v<[iu][1-9]\d*>"
@@ -98,7 +97,6 @@ highlight default link zigConditional Conditional
 highlight default link zigComparatorWord Keyword
 highlight default link zigRepeat Repeat
 highlight default link zigSpecial Special
-highlight default link zigVarDecl Function
 highlight default link zigPreProc PreProc
 highlight default link zigException Exception