feat: Add a mime::APPLICATION_XML constant

[CosmicHorrorDev]

There's already TEXT_XML, so adding APPLICATION_XML seems fitting considering that it's recommended over text/xml. Per RFC 7303 Section 4.1

Compared to [RFC2376] or [RFC3023], this specification alters the handling of character encoding of text/xml and text/xml-external-parsed-entity, treating them no differently from the respective application/ types. However, application/xml and application/xml-external-parsed-entity are still RECOMMENDED, to avoid possible confusion based on the earlier distinction. The former confusion around ...

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Found after opening, but this looks identical to (closed by author) #127

[mrhrzg]

I strongly second this. Please merge this.