Don't match a language prefix that is not a sublanguage in lang()

[youdie006]

XPath 1.0 lang() is true when xml:lang equals the argument or is a sublanguage of it (the argument followed by -), ignoring case. compare_language only checks for a prefix, so lang('en') also matches xml:lang="eng":

query	master	this PR	libxml2
//p[lang('en')] over en, eng, EN-us, e	en, eng, EN-us	en, EN-us	en, EN-us
//p[lang('e')]	all four	e	e
//p[lang('en-US')]	EN-us	EN-us	EN-us

https://www.w3.org/TR/xpath-10/#function-lang

The lang function returns true or false depending on whether the language of the context node as specified by xml:lang attributes is the same as or is a sublanguage of the language specified by the argument string. The language of the context node is determined by the value of the xml:lang attribute on the context node, or, if the context node has no xml:lang attribute, by the value of the xml:lang attribute on the nearest ancestor of the context node that has an xml:lang attribute. If there is no such attribute, then lang returns false. If there is such an attribute, then lang returns true if the attribute value is equal to the argument ignoring case, or if there is some suffix starting with - such that the attribute value is equal to the argument ignoring that suffix of the attribute value and ignoring case. For example, lang("en") would return true if the context node is any of these five elements:

<para xml:lang="en"/>
<div xml:lang="en"><para/></div>
<para xml:lang="EN"/>
<para xml:lang="en-us"/>

The new test sits next to test_lang; it fails on master and passes here. ruby -Ilib test/run.rb passes on Ruby 3.4 (also with --enable-frozen-string-literal), and the lang tests pass on 2.7.

Written with AI assistance (Claude); I have reviewed the change.

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[kou]

Good catch.

I've added the lang() part of XPath 1.0 specification to the PR description and merged this.