feat(css/compat): Add Compiler (#6626)

**Description:**

Visitor patterns are inherently slow, so I decided to merge all compatibility passes into a single pass.

crates/swc_css_compat/Cargo.toml

@@ -13,6 +13,7 @@ version = "0.16.0"
 bench = false
 
 [dependencies]
+bitflags = "1.3.2"
 once_cell = "1.10.0"
 serde = {version = "1.0.118", features = ["derive"]}
 serde_json = "1.0.61"

crates/swc_css_compat/src/compiler/custom_media.rs

@@ -0,0 +1 @@
+

crates/swc_css_compat/src/compiler/mod.rs

@@ -0,0 +1,26 @@
+use swc_css_visit::VisitMut;
+
+use crate::feature::Features;
+
+mod custom_media;
+
+/// Compiles a modern CSS file to a CSS file which works with old browsers.
+#[derive(Debug)]
+pub struct Compiler {
+    #[allow(unused)]
+    c: Config,
+}
+
+#[derive(Debug)]
+pub struct Config {
+    /// The list of features to **process**.
+    pub process: Features,
+}
+
+impl Compiler {
+    pub fn new(config: Config) -> Self {
+        Self { c: config }
+    }
+}
+
+impl VisitMut for Compiler {}

crates/swc_css_compat/src/feature.rs

@@ -0,0 +1,7 @@
+use bitflags::bitflags;
+
+bitflags! {
+    pub struct Features: u64 {
+        const NESTING = 0b00000001;
+    }
+}

crates/swc_css_compat/src/lib.rs

@@ -1,4 +1,6 @@
 #![feature(box_patterns)]
 #![allow(clippy::vec_box)]
 
+pub mod compiler;
+pub mod feature;
 pub mod nesting;