Remove error-prone source_id from TypeEngine::insert() and make inserting API more robust and less verbose

[ironcev]

In the TypeEngine::insert() method, the source_id parameter must be correctly provided in order for the LSP garbage collection to work properly.

The current signature of insert() is quite error-prone in that regard and verbose in many cases (e.g. inserting built-in types). The caller has to choose the valid source_id (or None) although the choice is in most of the cases determined by the type being inserted. E.g. if we are inserting an enum the source_id must always come from the enum declaration span. (This is something we will very likely change in #6603, but until then, inserting an enum should always follow the same logic for picking the source_id no matter where in code we insert a TypeInfo::Enum.)

Currently, since we are inserting same types at various places in code, it is only the convention that makes the source_id always defined in the same way for a particular type.

Therefore, the proposal is to make the insert() API robust in regard to source_id and less verbose at the same time by:

• removing source_id it from the insert() method and infer it based on the inserted type.
• provide convenience methods for inserting common types like Unknown or built-in types.