GH-1587: refactor embeddings module

[alanakbik]

Closes #1587

This PR splits the embeddings.py into several submodules organized in an embeddings/ folder. The submodules are:

• token.py for all TokenEmbeddings classes
• document.py for all DocumentEmbeddings classes
• image.py for all ImageEmbeddings classes
• legacy.py for embeddings that are now deprecated
• base.py for remaining basic classes

All embeddings are still exposed through the embeddings package, so the command to load them doesn't change, e.g.:

from flair.embeddings import FlairEmbeddings
embeddings = FlairEmbeddings('news-forward')

so specifying the submodule is not needed.