feat: Add ESP8266 support

* Added support for ESP8266
* Removed previous Arduino defines in favor
  of GTEST_OS_* syntax
* Improved PlatformIO library file to be more
  stable when consumed in client applications.

library.json

@@ -11,7 +11,8 @@
   "version": "1.8.1",
   "frameworks": "arduino",
   "platforms": [
-        "espressif32"
+        "espressif32",
+        "espressif8266"
   ],
   "export": {
         "include": [
@@ -38,6 +39,7 @@
             "googletest/m4",
             "googletest/make",
             "googletest/msvc",
+            "googletest/samples",
             "googletest/scripts",
             "googletest/src/gtest-all.cc",
             "googletest/src/gtest_main.cc",
@@ -54,6 +56,24 @@
             "-Igooglemock",
             "-Igoogletest/include",
             "-Igoogletest"
+        ],
+        "srcFilter": [
+          "+<*>",
+          "-<.git/>",
+          "-<googlemock>",
+          "-<googlemock/test/>",
+          "-<googlemock/src>",
+          "+<googlemock/src/gmock-all.cc>",
+          "+<googletest/src/gtest-all.cc>",
+          "+<googlemock/src/gmock_main.cc>",
+          "-<googletest>",
+          "-<googletest/codegear/>",
+          "-<googletest/samples>",
+          "-<googletest/test/>",
+          "-<googletest/xcode>",
+          "-<googletest/src>",
+          "+<googletest/src/gtest-all.cc>",
+          "+<googletest/src/gtest_main.cc>"
         ]
   }
 }