Re-add getAvailableLanguages, getAvailableThemes, languageIsAvailable and themeIsAvailable

[s3ththompson]

This PR re-adds getAvailableLanguages, getAvailableThemes, languageIsAvailable and themeIsAvailable and updates their behavior to correctly read all languages/themes, rather than just currently loaded ones.

Even though individual languages and themes are now lazy-loaded by Shiki, these methods allow an application to introspect the possible assets available for lazy-loading.

This functionality is essential for an application to present a dropdown list of highlighting options to a user, and to validate user-selected languages/themes.

I know you just released a new major version without these methods, but it would not be a breaking change to re-add them. I hope you'll consider this, despite the churn. Thanks!

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As an implementation detail, this PR differentiates internally between base languages (e.g. javascript) and aliases (e.g. javascript and js). getAvailableLanguages returns base languages, while languageIsAvailable checks against all aliases.

(This PR also fixes a small nit: symfony/process was not required as a dependency.)

[riasvdv]

Seems like the tests on older PHP versions are failing, can you check?

[s3ththompson]

Oops, looks like symfony/process needs to be ^5.4|^6.4|^7.1 to support all the way back to PHP 7.4. Should be fixed now!

[riasvdv]

Thanks!