Document changes since v1.2.0:
- Replace obsolete registerv-make with cl-defstruct for Emacs 30
- Fix pdf-virtual page link filtering
- Resolve native-comp warnings across multiple files
- Add doas support in autobuild for Alpine/OpenBSD
- Move CI to GitHub Actions

The registerv-make function was deprecated in Emacs 27.1. Replace it
with a proper cl-defstruct (pdf-view-register) and cl-defmethod
implementations for register-val-jump-to, register-val-describe,
and register-val-insert.

Add tests for the register API used by pdf-view:
- pdf-view-registerv-make-creates-register: verifies register creation
- pdf-view-register-val-insert: verifies insert method works
- pdf-view-position-to-register-stores-position: verifies store/retrieve

Fixes: #333

Reformat docstrings in pdf-view--bounding-box-to-slice,
pdf-document-common-bounding-box, and pdf-view-set-slice-common-bounding-box
to be under 80 characters wide and use proper quoting conventions.

Ref #333

Add declare-function statements for image-mode-window-get,
image-set-window-vscroll, and image-set-window-hscroll to silence
native compilation warnings about image-mode functions.

Ref #333

Add declare-function statements for pdf-util-frame-scale-factor,
pdf-util-hexcolor, pdf-util-munch-file, pdf-util-highlight-regexp-in-string,
and pdf-view-buffer-file-name to silence native compilation warnings.

Ref #333

Add declare-function statements for pdf-view-desired-image-size,
pdf-view-create-page, and image-mode-window-get to silence native
compilation warnings about functions not known to be defined.

Ref #333

Alpine Linux deprecated sudo in favor of doas. Add doas support to
exec_privileged() so autobuild works on Alpine without requiring users
to install and configure sudo.

Fixes #323