Enhancement: enrich csv export

[sfxn]

I find the "prompts to file" feature very useful but would be happy about these improvements:

1. add an index column. When using Combinatorial generation with several iterations, it would be helpful to have an index in the CSV, representing the nth iteration of the very image that was created. I myself use [generation_number] in the Images filename pattern. So when comparing good with bad results, it's easier to identify the row that matches the generated image.
2. for each variant add a separate column that represents its specific value that has been used within that specific prompt.

When using one random sampler and two combinatorial samplers within the positive prompt, the result might look like this:

index	positive_prompt	negative_prompt	variant_1	variant_2	variant_3
0	a {~hot|cold|mild} {summer|spring} is coming to {mexico|germany}	ugly	{~hot|cold|mild}	{summer|spring}	{mexico|germany}
1	a mild summer is coming to mexico	ugly	mild	summer	mexico
2	a hot summer is coming to germany	ugly	hot	summer	germany
3	a hot spring is coming to mexico	ugly	hot	spring	mexico
4	a cold spring is coming to germany	ugly	cold	spring	germany