add app.setLocale() (#11469)

* infrastructure for setLocale via klang

* add documentation for setLocale

* add test for setLocale

* fix spec

* add spec and update docs

* fix carriage feeds on windows

* SetLocale() sets LC_ALL on Linux

* in SetLocale() on Linux, use g_setenv()

* fix tyop: '#ifdef OSX_POSIX'

* make the linter happy

* improvements from review

spec/fixtures/api/locale-check/main.js

@@ -0,0 +1,10 @@
+const {app} = require('electron')
+
+app.on('ready', () => {
+  process.stdout.write(app.getLocale())
+  process.stdout.end()
+
+  setImmediate(() => {
+    app.quit()
+  })
+})