improve filename template

The default filename template for saving media files uses inconsistent casing: `ComfyUI_00001.png`, `ComfyUI_00001.webm`. That's CamelCase plus snake_case.

Technically, there's a strange trailing suffix in the basename: `ComfyUI_00001_.png` with an extra underscore just before `.png`.

Fortunately, train-case is more consistent and more ergonomic: `comfyui-00001.png`, `comfyui-00001.webm`. No need to involve the Shift key modifier. Less jarrring for the eyes, less taxing on the hands.

Can we please adjust the save nodes to apply train-case by default?

As well, I propose that single frame image save nodes use the default template `comfyui-img-xxxxx.png`, whereas animated save nodes use the template `comfyui-vid-xxxxx.webm`, `comfyui-vid-xxxxx.gif`. As opposed to a generic `comfyui` prefix. The current default prefix results in interleaved, mixed mimetypes when sorting by file columns other than mimetype, such as filename or recency. By including the static vs. animated nature of the media in the prefix, we are rewarded with a faster way to jump through large AI collections.