LibRegex: Hide the Unicode allocator override behind a feature

In a similar fashion to 46912fce8d.

In this case, a future commit will make LibURL depend on
LibRegex's Rust module, so we need to apply the same change here
too.

Libraries/LibRegex/CMakeLists.txt

@@ -5,7 +5,7 @@ set(SOURCES
 
 ladybird_lib(LibRegex regex EXPLICIT_SYMBOL_EXPORT)
 
-import_rust_crate(MANIFEST_PATH Rust/Cargo.toml CRATE_NAME libregex_rust FFI_HEADER RustFFI.h)
+import_rust_crate(MANIFEST_PATH Rust/Cargo.toml CRATE_NAME libregex_rust FEATURES allocator FFI_HEADER RustFFI.h)
 target_link_libraries(LibRegex PRIVATE libregex_rust LibUnicode)
 if ((LINUX OR BSD) AND NOT BUILD_SHARED_LIBS)
     target_link_options(LibRegex INTERFACE LINKER:--allow-multiple-definition)

Libraries/LibRegex/Rust/Cargo.toml

@@ -11,5 +11,9 @@ crate-type = ["staticlib"]
 [dependencies]
 libunicode_rust = { path = "../../LibUnicode/Rust", default-features = false }
 
+[features]
+default = []
+allocator = []
+
 [build-dependencies]
 cbindgen = "0.29"

Libraries/LibRegex/Rust/src/lib.rs

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#[cfg(feature = "allocator")]
 #[path = "../../../RustAllocator.rs"]
 mod rust_allocator;