Refactor background page to use Typescript

Also, this CL adds clean-terminal-webpack-plugin to make it easier to
debug Typescript errors while developing the extension.

Bug: translateselectedtext:15
Change-Id: If9b97cb7508859e2e05f5dc82940808fd935bf1a
diff --git a/src/common/sessionStorage.ts b/src/common/sessionStorage.ts
new file mode 100644
index 0000000..450e466
--- /dev/null
+++ b/src/common/sessionStorage.ts
@@ -0,0 +1,7 @@
+// #!if ['chromium_mv3', 'edge_mv3'].includes(browser_target)
+import ExtSessionStorage from './sessionStorage_mv3'
+// #!else
+import ExtSessionStorage from './sessionStorage_mv2'
+// #!endif
+
+export default ExtSessionStorage;