allow the crate to be build as rust library

[ju6ge]

this enables git import of r-polars in other rust based R libraries

Cargo.toml

[package]
name = "test-import"
version = "0.1.0"
edition = "2021"

[lib]
crate-type = ["rlib", "staticlib"]

[dependencies]
r-polars = { git = "https://github.com/ju6ge/r-polars", branch = "feature/enable-rust-git-import" }
extendr-api = { git = "https://github.com/extendr/extendr", rev = "1895bfc8ee22347665900caa0e48da4f0b13232f" }

lib.rs

use r_polars::rdataframe::RPolarsDataFrame;
use extendr_api::prelude::*;

#[extendr]
/// @export
pub fn export_dataframe() -> RPolarsDataFrame {
    RPolarsDataFrame::default()
}

extendr_module! {
    mod polars_export;
    fn export_dataframe;
}

[etiennebacher]

Thanks @ju6ge, that looks good to me and the change is very simple. I don't see why this would be a problem but I'm no expert on this, so I'll let @eitsupi validate and merge if it's ok