Add validation for image manipulation names to not be numeric

[shbchk]

I prefer to name my image manipulations by size. Like, 1600 for 1600px width.

But in some cases (I'm not sure how to reproduce) they just do not work in templates.
I mean, {meta_ogimage:1600} in template renders as {meta_ogimage:1600} in html.

I tried to rename image manipulation to i1600 and it worked, now in html I see link https://domain.local/images/_i1600/image.jpg instead of EE tag.

But, there is another problem now: image folders do not get new names after renaming image manipulation name, and that link points to a non-existing folder.

[intoeetive]

Using numbers as modifier (in this case, manipulation name) is not recommended - is there is overlap with entry ID or some other ID, the system might behave in an unexpected way. Always use a non-numeric prefix (I'll classify this issue as feature request to add validation for this)

And correct, there is no way to rename the manipulation and keep the files, - it's a filesystem-based thing, so the folder name will be changed.

You now need to synchronize your upload directory

[shbchk]

Okay, thanks, Yuri!

[intoeetive]

This will be implemented with merge of #3367 (EE6) and #3366 (EE7)