bug(knitr): extendrsrc doesn't support new `#[extendr(default="")]` syntax

repro: ```{extendrs use_dev_extendr=TRUE} #[extendr] fn check_default(#[extendr(default = "NULL")] x: Robj) -> bool { x.is_null() } ``` ``` ✔ Compiling Rust extendr code chunk... ℹ build directory: '/private/var/folders/wd/xq999jjj3bx2w8cpg7lkfxlm0000gn/T/RtmpryXs54/file1184525e98ee8' Error in `extract_meta()`: ! Rust code contains invalid attribute macros. • ✖ No valid `fn` or `impl` block found in the following sample: • #[extendr] Backtrace: ```