Autolink: fix invalid string when email is followed by a multibyte character

[goulvench]

sd_autolink__email was using raw isalnum to detect email addresses, but isalnum can return true for non-ASCII bytes. This caused autolink to incorrectly capture trailing multibyte characters (e.g. contact@example.com中 would include the lead byte of 中 at the end of the email).

markdown.c already had a _isalnum helper that combines isalnum with an explicit c < 0x7f guard for exactly this reason. This PR moves that helper to autolink.h (which is included by both files via markdown.h) and uses it consistently in sd_autolink__email.

Fixes #811