MIME: added audio/x-wav type.

Chrome and other browsers implementing Opaque Request Blocking block
`<audio>` elements from playing if it cannot guess it is an audio file
and the Content-Type header does not say it is that [1]. This is
currently the case for WAV files served by Nginx instances configured
to use the included `mime.types` file. Chrome is unable to
guess a file's type if the server responds with 206 Partial Content,
which is typically the case for WAV files.

Unfortunately, there is no official MIME type for WAV files. I decided
to go with `audio/x-wav` since it is used by Apache HTTPD [2] and the
media-types Debian package [3].

WAV files are supported by browsers used by 98.01% of the world's
population [4].

[1]: https://groups.google.com/a/chromium.org/g/blink-dev/c/ScjhKz3Z6U4/m/5i_0V7ogAwAJ
[2]: 4a9cd1fccf/docs/conf/mime.types (L1526)
[3]: 8b715798bb/mime.types (L1845)
[4]: https://caniuse.com/wav

conf/mime.types

@@ -83,6 +83,7 @@ types {
     audio/ogg                                        ogg;
     audio/x-m4a                                      m4a;
     audio/x-realaudio                                ra;
+    audio/x-wav                                      wav;
 
     video/3gpp                                       3gpp 3gp;
     video/mp2t                                       ts;