refactor bridge plugin into virtual module

packages/shim/src/fs/virtual-files.js

@@ -0,0 +1,23 @@
+// Virtual plugin source served from memory; the fs shim's read path checks here before disk.
+
+function normalize(p) {
+  return (p || "").replace(/\\/g, "/").replace(/^\/+/, "").replace(/\/+$/, "");
+}
+
+const virtualFiles = new Map();
+
+export function setVirtualFile(path, content) {
+  virtualFiles.set(normalize(path), content);
+}
+
+export function removeVirtualFile(path) {
+  virtualFiles.delete(normalize(path));
+}
+
+export function getVirtualFile(path) {
+  return virtualFiles.get(normalize(path));
+}
+
+export function hasVirtualFile(path) {
+  return virtualFiles.has(normalize(path));
+}