Permit `include!(…)` in pattern position for completeness's sake

At the time of writing, built-in attr include is not permitted to expand into a pattern context (its expander doesn't override make_pat):

fn main() {
    match () { include!("/dev/null") => {} } //~ ERROR non-pattern macro in pattern position: include
}

For nothing but "completeness" (item, expr, pat trifecta) and for consistency with include_str and include_bytes I propose that we permit them in this position.

include's expander would then parse a PatternNoTopGuard (as defined in RFC 3637), i.e., top-level or-patterns are permitted (and guards aren't – ignoring guard_patterns for now). Meaning let include!("file"); with file containing false | true would be permitted despite let false | true; not being valid (requiring parentheses) since the macro expansion would act as invisible delimiters. Leading vert / bar would be permitted as well, naturally.

Lastly, don't strip shebang or frontmatter (meaning they'll lead to an error since the pattern parser won't accept it). Edit: Well, it depends on the outcome of https://github.com/rust-lang/rust/pull/146377.

Start with the built-in include expander and its missing make_pat handling, then review the PatternNoTopGuard requirements from RFC 3637 and the related PR #146377. Done means include! can expand in pattern position with the specified or-pattern and leading-bar behavior, while shebang/frontmatter handling follows the outcome of that PR.