Direct SVG rendering (#12157)

Introduce 'make svg' which calls a node script that compiles svg files
to `public/img/svg`. These files are vendored to not create a dependency
on Node for the backend build.

On the frontend side, configure webpack using `raw-loader` so SVGs can
be imported as string.

Also moved our existing SVGs to web_src/svg for consistency.

Fixes: https://github.com/go-gitea/gitea/issues/11618

web_src/js/markdown/anchors.js

@@ -1,4 +1,4 @@
-import {svg} from '../utils.js';
+import {svg} from '../svg.js';
 
 const headingSelector = '.markdown h1, .markdown h2, .markdown h3, .markdown h4, .markdown h5, .markdown h6';