Fix support for prerelease exits using the new .changeset/pre layout (#711)

* Fix prerelease exit mode selection

* Improve prerelease exit changeset wording

src/readChangesetState.test.ts

@@ -0,0 +1,40 @@
+import { createFixture } from "fs-fixture";
+import { describe, expect, it } from "vitest";
+import readChangesetState from "./readChangesetState.ts";
+
+const changeset = `---
+"pkg-a": patch
+---
+
+Fix a bug.
+`;
+
+describe("readChangesetState", () => {
+  it("filters versioned changesets in prerelease mode", async () => {
+    await using fixture = await createFixture({
+      ".changeset/pre.json": JSON.stringify({ mode: "pre", tag: "next" }),
+      ".changeset/pre/versioned.md": changeset,
+      ".changeset/pending.md": changeset,
+    });
+
+    const state = await readChangesetState(fixture.path);
+
+    expect(state.changesets.map((changeset) => changeset.id)).toEqual([
+      "pending",
+    ]);
+  });
+
+  it("includes versioned changesets when exiting prerelease mode", async () => {
+    await using fixture = await createFixture({
+      ".changeset/pre.json": JSON.stringify({ mode: "exit", tag: "next" }),
+      ".changeset/pre/versioned.md": changeset,
+    });
+
+    const state = await readChangesetState(fixture.path);
+
+    expect(state.preState).toBeUndefined();
+    expect(state.changesets.map((changeset) => changeset.id)).toEqual([
+      "pre/versioned",
+    ]);
+  });
+});