chore: tighten package boundary + clean rebuild

Two changes that prevent long-term shape drift once published:

- package.json `exports`: gate the public surface to the documented
  default import (`postcss-calc` → `./dist/src/index.js`) plus
  `./package.json` for tooling that reads it. Modern Node and
  bundlers will reject deep imports like `postcss-calc/dist/...`
  or `postcss-calc/src/...`, preventing consumers from accidentally
  locking themselves to the internal AST / tokenizer modules.
- Add a `prebuild` script that `rm -rf dist`s before each tsc emit.
  Without this, stale build output from earlier directory layouts
  (e.g., a previous `dist/src/v11/` tree) survives across rebuilds
  and ships in the npm tarball.

Also sync pnpm-lock.yaml — the previous commit dropped
postcss-selector-parser from dependencies but didn't regenerate
the lockfile, which would break `pnpm install --frozen-lockfile`
on the maintainer's CI.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dkryaklin

package.json

@@ -16,6 +16,13 @@
   },
   "main": "dist/src/index.js",
   "types": "dist/src/index.d.ts",
+  "exports": {
+    ".": {
+      "types": "./dist/src/index.d.ts",
+      "default": "./dist/src/index.js"
+    },
+    "./package.json": "./package.json"
+  },
   "files": [
     "dist",
     "src",
@@ -24,6 +31,7 @@
   ],
   "scripts": {
     "prepare": "pnpm run build",
+    "prebuild": "rm -rf dist",
     "build": "tsc -p tsconfig.build.json",
     "build:jison": "jison ./parser.jison -o src/parser.js",
     "lint": "eslint . && tsc",