Update var to const where applicable

This no longer builds with the latest version of Zig.

See: ziglang/zig#18017

Author: linusg

build.zig

@@ -8,7 +8,7 @@ pub fn build(b: *std.build.Builder) void {
         .dependencies = &.{},
     });
 
-    var main_tests = b.addTest(.{
+    const main_tests = b.addTest(.{
         .root_source_file = .{ .path = "src/main.zig" },
         .optimize = optimize,
     });

src/tokenizer.zig

@@ -297,7 +297,7 @@ test "save/restore tokenization" {
     , null);
 
     const id0 = (try tokenizer.next()) orelse return error.MissingToken;
-    var state = tokenizer.saveState();
+    const state = tokenizer.saveState();
     const ws0 = (try tokenizer.next()) orelse return error.MissingToken;
     tokenizer.restoreState(state);
     const ws1 = (try tokenizer.next()) orelse return error.MissingToken;