Introduce GiteaLocaleNumber custom element to handle number localization on pages. (#23861)

Follow #21429 & #22861

Use `<gitea-locale-number>` instead of backend `PrettyNumber`. All old
`PrettyNumber` related functions are removed. A lot of code could be
simplified.

And some functions haven't been used for long time (dead code), so they
are also removed by the way (eg: `SplitStringAtRuneN`, `Dedent`)

This PR only tries to improve the `PrettyNumber` rendering problem, it
doesn't touch the "plural" problem.

Screenshot:


![image](https://user-images.githubusercontent.com/2114189/229290804-1f63db65-1e34-4a54-84ba-e00b44331b17.png)


![image](https://user-images.githubusercontent.com/2114189/229290911-c88dea00-b11d-48dd-accb-9f52edd73ce4.png)

webpack.config.js

@@ -60,7 +60,7 @@ export default {
       fileURLToPath(new URL('web_src/css/index.css', import.meta.url)),
     ],
     webcomponents: [
-      fileURLToPath(new URL('web_src/js/webcomponents/GiteaOriginUrl.js', import.meta.url)),
+      fileURLToPath(new URL('web_src/js/webcomponents/webcomponents.js', import.meta.url)),
     ],
     swagger: [
       fileURLToPath(new URL('web_src/js/standalone/swagger.js', import.meta.url)),