[Lexer] Add support for \ProvidesExpl(Class|File)

[Sirraide]

Summary of additions and changes

In addition to \ProvidesExplPackage, LaTeX3 also introduces \ProvidesExplClass
and \ProvidesExplFile, which also have the effect of enabling LaTeX3 syntax. This
pr adds support for the latter two.

See the LaTeX 3 interfaces manual, § 2.1 ‘Using the LaTeX3 modules’.

How to test this pull request

\NeedsTeXFormat{LaTeX2e}
\ProvidesExplClass{myclass}{2024/06/08}{v1.0}{A class implemented in LaTeX3}
\cs_new:Npn \__my_internal_class_command:n #1 { Hello, ~ #1 ! }

• Updated the documentation, or no update necessary
• Added tests, or no tests necessary

(Ticked the second box on the grounds that there were no tests for \ProvidesExplPackage either...)

[PHPirates]

Thanks!