Clear auto-import specifier cache when module specifier preferences change

[lsh4711]

Fixes #64820

Registry.specifierCache was only cleared when autoImportSpecifierExcludeRegexes changed, so a change to importModuleSpecifierPreference or importModuleSpecifierEnding kept returning specifiers computed with the old preferences for files that were already open.

This compares all preferences returned by ModuleSpecifierPreferences() (the ones passed to GetModuleSpecifier) through a new modulespecifiers.UserPreferences.Equal, and clears the cache only when they differ.

Added TestUserPreferencesEqual, and TestAutoImportModuleSpecifierAfterUserPreferenceChange, which changes autoImportSpecifierExcludeRegexes, importModuleSpecifierEnding and importModuleSpecifierPreference one at a time on an open file and checks the auto-import specifier after each change.

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[Ryan Cavanaugh (RyanCavanaugh)]

This clears the cache on every user preference change

This is a strong claim that isn't well-supported by the code. It's allowed for the editor to re-send config even in the absence of a change, and it's easy to set up your editor to allow toggling preferences a lot for whatever reason (e.g. inlay hints). Without stronger justification than "it's rare so it's OK" I'd really prefer this just be done correctly (check the preferences values that matter).

[lsh4711]

Thanks, that's fair. I changed it to clear the cache only when the preferences returned by ModuleSpecifierPreferences() change, which are the ones passed to GetModuleSpecifier. I added an Equal method next to modulespecifiers.UserPreferences (following bucketBuildPreferences.Equal), so a field added to that struct is next to its comparison, and a unit test for it.