ENH: Add a rotate command, per discussion

[hwine]

Adds a rotate sub command to rotate the specified pages by the specified degrees. Design as discussed in #119 (exceptions noted at end).

Current status:

• all tests pass on GitHub
• new code has 83% coverage (missing a test of an exception handler)
• new tests have 100% coverage

pdfly rotate --help

 Usage: pdfly rotate [OPTIONS] FILENAME DEGREES [PGRGS]

 Rotate specified pages by the specified amount
 Example:
     pdfly rotate --output output.pdf input.pdf 90
         Rotate all pages by 90 degrees (clockwise)

 A file not followed by a page range (PGRGS) means all the pages of the file.

 PAGE RANGES are like Python slices.

         Remember, page indices start with zero.

         Page range expression examples:

             :     all pages.                   -1    last page.
             22    just the 23rd page.          :-1   all but the last page.
             0:3   the first three pages.       -2    second-to-last page.
             :3    the first three pages.       -2:   last two pages.
             5:    from the sixth page onward.  -3:-1 third & second to last.

         The third, "stride" or "step" number is also recognized.

             ::2       0 2 4 ... to the end.    3:0:-1    3 2 1 but not 0.
             1:10:2    1 3 5 7 9                2::-1     2 1 0.
             ::-1      all pages in reverse order.

╭─ Arguments ────────────────────────────────────────────────────────────────────────────────────────────────────╮
│ *    filename      FILE     [default: None] [required]                                                         │
│ *    degrees       INTEGER  degrees to rotate [default: None] [required]                                       │
│      pgrgs         [PGRGS]  page range [default: :]                                                            │
╰────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
╭─ Options ──────────────────────────────────────────────────────────────────────────────────────────────────────╮
│ *  --output  -o      PATH  [default: None] [required]                                                          │
│    --help                  Show this message and exit.                                                         │
╰────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯

The not-implemented-as-discussed elements are:

• in-place modifications. I'm disinclined to add this as:
  • there doesn't seem to be consensus on this with other features (ref)
  • the current approach in other commands does not appear to address "huge file" issues (i.e. running out of memory to hold both documents)
  • the the current overwrite approach might lead to data loss if there is insufficient disk space. (Perhaps not - the file might be guaranteed to be the same file size, but that requires investigation.)

[Lucas-C]

The PR Title Check / Title check GitHub Actions rule is failing:
could you please add a ENH prefix to this PR title, please?
cf. https://pdfly.readthedocs.io/en/latest/dev/intro.html#commit-messages

Also could you add a mention of this nice addition into CHANGELOG.md as part of this PR please?

[Lucas-C]

Thank you for your contribution @hwine 👍

I made a few feedback comments, but it's great code overall!

[Lucas-C]

@all-contributors please add @hwine for code: 17ba644

[hwine]

@Lucas-C thanks for catching my "oops" -- I believe I've addressed all of your comments

[Lucas-C]

This new command has been included in the latest 0.5.0 release: https://github.com/py-pdf/pdfly/releases/tag/0.5.0