add: `--overwrite` mode

[metaist]

Very often I have documents A, B, C where I want to produce A += B + C, B += C.

So I could run pdfmerge --append A.pdf B.pdf C.pdf which would be the same as pdfmerge --output A.pdf A.pdf B.pdf C.pdf.

[metaist]

I like --append as a name, but what I really want is something like --update-- the first path is the output path.

# replace A.pdf with the first 10 pages
pdfmerge --update A.pdf[1..10] # == pdfmerge  A.pdf[1..10] --output A.pdf

# rotate all the pages in B.pdf clockwise
pdfmerge --update B.pdf[>] # == pdfmerge B.pdf[>] --output B.pdf

[metaist]

After a long conversation with ChatGPT, I actually think --overwrite / -w is a better name.