Remove linkprefix message, add $linkPrefixCharset

The existing "linkprefix" message is unlikely to be accurately
customized by message translators (as shown by the fact that, of the 10
distinct customizations prior to Iaa7eaa44 (which made them even more
complicated), 3 were broken or entirely ineffective, 1 was half
ineffective, and 2 more seem to have included the Latin-1 Supplement by
accident) or by local wiki admins. So, like linktrail before it, let's
move it out of the system messages and into a separate language
variable.

At the same time, let's make it a simple character set (like
$wgLegalTitleChars) rather than a complicated regular expression. The
complicated regex now lives in the parser.

This also adjusts the output of the API's action=query&meta=siteinfo and
adds an accessor parallel to the linkTrail accessor to Language.

Note the following changes that are not simply extracting the existing
charset from the linkprefix message for $linkPrefixCharset:
* The En message matched all non-ASCII UTF-8 characters by matching the
  component bytes (\\x80-\\xff). The new character set is equivalent.
* Various languages were identical to En and so have no $linkPrefixCharset
  set. These are: Ary Az Ce Ga Id Ka Kiu Km Ltg Mk Ms Ne Nn Ro Roa_tara Sc Si
  Sr_ec Sr_el Tl Tt_cyrl Tt_latn Ug_arab War
* Cu, Uk, and Udm are changed to match any number of „ or « in the prefix.
* Cv tried to include "«" that was redundant to the range \\x80-\\xff
  (see En comment). This was removed.
* Diq was entirely bogus, and so was removed.
* Gu included many additional UTF-8 characters that are redundant to the
  range \\x80-\\xff (see En comment). These were removed, and the
  resulting character set is equivalent to En.
* Mt has been broken since it was introduced in r37242. The charset used is
  equivalent to the broken regex.

Bug: 56031
Change-Id: I3369851b33113fc118a1bace38f3ac310cdd9725
Merge "Remove linkprefix message, add $linkPrefixCharset"