feat(packages): New scalebox package for arbitrary box re-scaling

[Omikhleia]

Closes #1178

EDIT: Also Closes #1035 (see comments below)

Box re-scaling via PDF transformation matrix:

\scalebox[xratio=<number>, yratio=<number>]{<content>}

Rationale

Both @ctrlcctrlv and @alerque in the above-mentioned issue said it should be proposed to the core distribution... So here it is, basic logic extracted from textsubsuper.sile and re-vamped as as standalone package.

Caveats

• It's only tested (see below) in LTR writing direction: Checking RTL and vertical writing directions is left as an exercise to readers who know how to use these.
• It's using the "hbox stealing trick": When/if Support migrating content in hbox building logic #1702 makes it, it is left as an exercise to adapt the code so that migrating contents are honored.
• It is also left as an exercise where to include the package documentation in the manual.

The first series of lines just demonstrate the basic feature. The second series ensures line stretching/shrinking is correctly applied. The very last line is just for the show.

Example code attached as .txt so it can be uploaded here:
feat-scalebox.txt

[Omikhleia]

Oops wrong file on first push... fixed and rebased. Sorry about it.

(But in case you look at the old commit hash... Yep, the rotate package doesn't like \hfill in content. I was looking at it out of curiosity, but it wasn't a fix, just quick tests... And I am not going to fix it (here), (anyway).)

[Omikhleia]

N.B. #1035 called for a \reflectbox (with also hints to achieve it via PDF matrix transform). This would just be a special case = \scalebox[xratio=-1,yratio=1]