[Sync EN] fix(mb-strlen): document invalid encoding ValueError and add shared snippets (#5418)

Commit: `https://github.com/php/doc-en/commit/8465ca6ec4bbcf79583ecd057860164dd9c7e385`
PR: `https://github.com/php/doc-en/pull/5418`

**Fichiers FR à mettre à jour**
- `language-snippets.ent`
- `reference/mbstring/functions/mb-encoding-aliases.xml`
- `reference/mbstring/functions/mb-internal-encoding.xml`
- `reference/mbstring/functions/mb-strlen.xml`
