convert : experimental support for --mmproj flag (ggml-org#13023)

* convert : experimental support for `--mmproj` flag

* fix bad ctrl+f replace

* fix style

* split into subclasses TextModel and VisionModel

* rename Mode --> ModelBase

* small fix

* correct CLIP_VISION arch name (because existing GGUF already use it)

* Apply suggestions from code review

Co-authored-by: compilade <[email protected]>

* fix Mistral3Model

* fix typo

Co-authored-by: compilade <[email protected]>

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Co-authored-by: compilade <[email protected]>

Author: ngxson