feat: add pkgbuild pager config

[shbernal]

Summary

• add a PkgbuildPager config option for yay -Gp output
• preserve current raw stdout behavior when the option is unset
• pipe generated PKGBUILD output through the configured command when set
• document the option and update shell completions

Example config:

PkgbuildPager = bat -pp --color=always -lPKGBUILD

Closes #2831

Tests

• go test ./...
• make test
• git diff --check

Note

This PR was prepared with assistance from OpenAI Codex. The changes were reviewed locally and validated with the tests above.