Reviewing metadata

[Janther]

This PR addresses how we interact with the getMetadata() and updateMetadata() functions.
Since every node calls these functions in order to retrieve the location information of the node and the comments contained by it.

All node files will have the same changes and the important changes are:

1. The introduction of the src/slang-nodes/SlangNode.ts which from now on starts behaving as a base class for all nodes.
2. The introduction of the src/slang-nodes/CommentNode.ts which from now on starts behaving as a base class for all comments.
3. The removal of src/slang-utils/metadata.ts since all of it's logic was moved into src/slang-nodes/SlangNode.ts as constructor or instance function.

To check the progress step by step, here are the main changes to look at commit by commit:

• fa46253 src/slang-utils/metadata.ts and src/types.d.ts
• e5fa3ed every file have the same change pattern.
• 7b09497 src/slang-utils/metadata.ts and src/types.d.ts.
• 4498ad2 added src/slang-nodes/SlangNode.ts and src/slang-nodes/CommentNode.ts and removed src/slang-utils/metadata.ts.
• 76056f3 small change in the function signature of updateMetadata in src/slang-nodes/SlangNode.ts