mime_db: New text/markdown MIME type and other MIME type polishing

- New text/markdown MIME type was added according to IANA registry.
- MIME types gained new extensions that weren't previously incl

mime_db: New text/markdown MIME type and other MIME type polishing

- New text/markdown MIME type was added according to IANA registry.
- MIME types gained new extensions that weren't previously included.
- Resource definition files will now open up with Pe by default.
- text/x-vcard was renamed to text/vcard, as it's now a registered IANA MIME type.
- text/x-source-code had its sniffing rules increased in priority,
so that it would get precedence over text/plain and its own sniffing rules.

Change-Id: I28af24de8c0e96ef39dfc67703a0884e8d4c842e
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4109
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

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