fix: keep every published file reachable, and test all four build artifacts

Two follow-ups to the dual packaging change (#628), found while running a
consumer-facing regression sweep against published 2.14.1.

An exports map is a closed list. Before #628 bowser had no exports map, so
every published file was reachable by subpath; afterwards LICENSE, README.md
and index.d.ts resolved to ERR_PACKAGE_PATH_NOT_EXPORTED. Nothing in the
documented API regressed, but `/// <reference types="bowser/index.d.ts" />`
and tooling that resolves the license by specifier both did. Add the four
non-code files to the map so the published surface matches 2.14.1 exactly,
and assert it in the package smoke test (25 -> 29 checks).

The acceptance suite asserted src/bowser.js and es5.js against the UA corpus
but not bundled.js or bowser.mjs. bowser.mjs is what the `import` condition
resolves to, so it is the file every modern ESM and bundler consumer runs,
and a build regression confined to it would have kept CI green. Assert all
four artifacts against the spec instead.

Verified by injecting a Chrome-parsing regression into bowser.mjs and into
bundled.js separately (each turns the suite red, labelled by artifact), and
by running the new smoke assertions against a pre-fix tarball (exit 1) and
this one (exit 0) on Node 12.16, 14, 18, 20 and 22.

package.json

@@ -71,6 +71,10 @@
     "./src/parser-platforms": "./src/parser-platforms.js",
     "./src/utils.js": "./src/utils.js",
     "./src/utils": "./src/utils.js",
+    "./LICENSE": "./LICENSE",
+    "./README.md": "./README.md",
+    "./index.d.ts": "./index.d.ts",
+    "./index.d.mts": "./index.d.mts",
     "./package.json": "./package.json"
   },
   "repository": {