ISO 216 document size constants

[bbrks]

Problem

I want to generate an A6 PDF and have to jump through a few hoops to get that document since, since there's only a small number of document size consts available

Proposed Solution

Include a full set of ISO 216 paper sizes ([A-C][0-10] + DL)

Alternatives Considered

Manually specifying Size{} - it's OK, but more ergonomic to use consts for the common doc sizes like A4 is used

Scope

template (Layer 3: builder API)

[bbrks]

I have a branch for this but cannot open PRs against this repo without being a contributor - I acknowledge the contributing guide suggests discussing on issue first.

https://github.com/gpdf-dev/gpdf/compare/main...bbrks:gpdf:feat/more_template_sizes?expand=1