Improve CheckDefaultBuilddir (AL1)

The current implementation of the parser skips over parameter expressions and turns them into short (without braces) versions of those variables. This means that cases where parameter expressions are used to modify things like pkgname and pkgver are still marked as using the default builddir.

Instead of implementing logic ourselfs to parse the AST for these expressions, switch to use the mvdan.sh/sh/v3/expand package that already has all this logic implemented.

Also add more acceptance tests for these cases, as well as tests for the linter and parser.