Fix favicon merging with customized icons (#67982)
### What
Support merging the static metadata file conventions `favicon.ico` with
other customized metadata icon from the module export.
### Why
`favicon.ico` should be displayed everywhere as it's the icon
representative of the website for all pages. Any customized `icon` in
metadata export `export const metadata` or `export function
generateMetadata()` shouldn't override the `favicon.ico` file
convention.
Fixes #55767
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Co-authored-by: hrmny <8845940+ForsakenHarmony@users.noreply.github.com>
