clang-format: [JS] test declared fields.

[mprobst]

Summary

TypeScript now supports declaring fields:

class Foo {
  declare field: string;
}

clang-format happens to already format this fine, so this change just
adds a regression test.

[MyDeveloperDay]

Thanks for this patch, this LGTM

clang/unittests/Format/FormatTestJS.cpp

verifyFormat("class Example {\n"
                "    }\n"
                "  }\n"
                "  #privateMethod() {\n"
                "    this.#privateMethod();  // infinite loop\n"
                "  }\n"
                "  static #staticPrivateMethod() {}\n");
 }
 
+TEST_F(FormatTestJS, DeclaredFields) {
+  verifyFormat("class Example {\n"
+               "  declare pub: string;\n"
+               "  declare private priv: string;\n"
+               "}\n");
+}
+
 } // namespace format
 } // end namespace clang