Simplified importer search and fixed a few bugs

The search for a matching importer had a few issues, see #3791. There were two different mechanisms to determine whether an importer accepts a specific file extension:
1. `aiImporterDesc::mFileExtensions`, which was forwarded to the UI via `BaseImporter::GetExtensionList()`.
2. `BaseImporter::CanRead()` when called with `checkSig == false`, which determines whether to actually use that importer.
Both were redundant and got out of sync repeatedly. I removed 2. completely and replaced it with 1., thereby syncing UI/import and shortening all `BaseImporter::CanRead()` implementations.

Further bugfixes:
- fixed glTF2 importer throwing exceptions when checking whether it can load a file
- removed `BaseImporter::SimpleExtensionCheck()` because it is no longer used and had a bug with case sensitivity

Since the `checkSig` parameter in `BaseImporter::CanRead()` is now useless, it can be removed completely. I’m not sure if this would break ABI compatiblity, so I’ll submit it with a later pull request.

code/AssetLib/ASE/ASELoader.cpp

@@ -97,19 +97,9 @@ ASEImporter::~ASEImporter() {
 
 // ------------------------------------------------------------------------------------------------
 // Returns whether the class can handle the format of the given file.
-bool ASEImporter::CanRead(const std::string &pFile, IOSystem *pIOHandler, bool cs) const {
-    // check file extension
-    const std::string extension = GetExtension(pFile);
-
-    if (extension == "ase" || extension == "ask") {
-        return true;
-    }
-
-    if ((!extension.length() || cs) && pIOHandler) {
-        const char *tokens[] = { "*3dsmax_asciiexport" };
-        return SearchFileHeaderForToken(pIOHandler, pFile, tokens, 1);
-    }
-    return false;
+bool ASEImporter::CanRead(const std::string &pFile, IOSystem *pIOHandler, bool /*checkSig*/) const {
+    static const char *tokens[] = { "*3dsmax_asciiexport" };
+    return SearchFileHeaderForToken(pIOHandler, pFile, tokens, std::size(tokens));
 }
 
 // ------------------------------------------------------------------------------------------------