parse unsafe extern blocks

[m4rch3n1ng]

fixes: #250

i couldn't find a way to properly resolve the conflict between unsafe extern "C" fn and unsafe extern "C" {} without adding them to the conflicts field for now. if you have an idea on how to do that better then i would like.

specified here, here and here

regarding #229:

diff

--- .tmp/list.txt	2025-02-28 16:16:12.002933806 +0100
+++ .tmp/list.txt.unsafe-extern-mod	2025-02-28 19:10:18.478498116 +0100
@@ -1,7 +1,6 @@
 tests/codegen/dst-offset.rs
 tests/codegen/sanitizer/cfi/emit-type-metadata-id-itanium-cxx-abi-user-defined-types.rs
 tests/codegen/function-arguments.rs
-tests/codegen/terminating-catchpad.rs
 tests/coverage/attr/nested.rs
 tests/coverage/coroutine.rs
 tests/coverage/coverage_attr_closure.rs
@@ -216,7 +215,6 @@
 tests/ui/parser/keyword-union-as-identifier.rs
 tests/ui/parser/parser-unicode-whitespace.rs
 tests/ui/parser/trait-plusequal-splitting.rs
-tests/ui/parser/unsafe-foreign-mod.rs
 tests/ui/pattern/issue-22546.rs
 tests/ui/pattern/usefulness/integer-ranges/issue-117648-overlapping_range_endpoints-false-positive.rs
 tests/ui/privacy/decl-macro-infinite-global-import-cycle-ice-64784.rs
@@ -230,8 +228,6 @@
 tests/ui/rust-2018/try-macro.rs
 tests/ui/rust-2018/uniform-paths/from-decl-macro.rs
 tests/ui/rust-2018/uniform-paths/macro-rules.rs
-tests/ui/rust-2024/unsafe-extern-blocks/extern-items.rs
-tests/ui/rust-2024/unsafe-extern-blocks/safe-items.rs
 tests/ui/simd/masked-load-store.rs
 tests/ui/sized/coinductive-1-gat.rs
 tests/ui/specialization/assoc-ty-graph-cycle.rs