add ability to retrieve current list of embedding trigger strings

This PR adds a new attributer to ldm.generate, `embedding_trigger_strings`:

```
gen = Generate(...)
strings = gen.embedding_trigger_strings
strings = gen.embedding_trigger_strings()
```

The trigger strings will change when the model is updated to show only
those strings which are compatible with the current
model. Dynamically-downloaded triggers from the HF Concepts Library
will only show up after they are used for the first time. However, the
full list of concepts available for download can be retrieved
programatically like this:

```
from ldm.invoke.concepts_lib import HuggingFAceConceptsLibrary
concepts = HuggingFaceConceptsLibrary()
trigger_strings = concepts.list_concepts()
```

ldm/invoke/args.py

@@ -751,6 +751,9 @@ class Args(object):
                 !fix applies upscaling/facefixing to a previously-generated image.
                 invoke> !fix 0000045.4829112.png -G1 -U4 -ft codeformer
 
+            *embeddings*
+                invoke> !triggers     -- return all trigger phrases contained in loaded embedding files
+
             *History manipulation*
             !fetch retrieves the command used to generate an earlier image. Provide
             a directory wildcard and the name of a file to write and all the commands