fix(bindings): correct Zig bindings to expose a language function

Instead of having users declare the extern function themselves, they can
pass in the language to `Language.create` in the zig bindings. If they
really want, they can always opt into the `extern fn tree_sitter_LANG()
*const ts.Language` approach.

cli/src/templates/root.zig

@@ -0,0 +1,19 @@
+const testing = @import("std").testing;
+
+const ts = @import("tree-sitter");
+const Language = ts.Language;
+const Parser = ts.Parser;
+
+pub extern fn tree_sitter_PARSER_NAME() callconv(.C) *const Language;
+
+pub export fn language() *const Language {
+    return tree_sitter_PARSER_NAME();
+}
+
+test "can load grammar" {
+    const parser = Parser.create();
+    defer parser.destroy();
+    try testing.expectEqual(parser.setLanguage(language()), void{});
+    try testing.expectEqual(parser.getLanguage(), tree_sitter_PARSER_NAME());
+}
+