Handle null instance and fall back to SFC filename in Vue error handler - #679

Fixes #678.

Two fixes to the @appsignal/vue error handler when used with Vue 3:

Null instance crash. Vue 3 invokes app.config.errorHandler(err, instance, info) with instance: null for errors raised outside a component context (e.g. in watchers, or lifecycle hooks after unmount). The handler dereferenced the instance unguarded and threw a TypeError of its own, so the original error never reached AppSignal or console.error. It now reports these as [unknown Vue component].

Component name resolution on Vue 3. The $vnode branch is Vue 2 API (always undefined on Vue 3), $options.name requires an explicit name option, and __name only exists for <script setup> components. For classic Options API SFCs none of these match, so every error grouped under [unknown Vue component]. The handler now falls back to the basename of $options.__file, which vue-loader sets in development and, when the app enables its exposeFilename option, as a basename in production builds. When __file is absent the existing [unknown Vue component] fallback is preserved, so behaviour only changes for apps that opt in.

Includes a mono changeset (bump: patch, type: fix) and test coverage for the new branches: __file with a full path (development), basename-only __file (production), explicit name taking precedence, a null instance, and empty options. Verified locally with jest in packages/vue (8 passing) and prettier --check.