Support [build].watch for theme app extensions

[erikmay]

Summary

Theme app extensions currently use the base BaseSchema (which has no build field) and don't implement devSessionWatchConfig. As a result, any [build] block in shopify.extension.toml is silently stripped by zod, and the CLI's dev watcher falls back to globbing **/* under the extension directory.

This is painful when an external build tool (Vite, esbuild, tsc --watch) writes multiple output files per build — each write fires an "Extension changed" event, triggering redundant theme-check and bundle cycles. A single manual bun run theme-extension:build during shopify app dev can produce 8+ "Extension changed" events and 2 full bundle cycles.

This PR makes the theme spec support [build].watch the same way function extensions already do:

• Extend BaseSchema into ThemeExtensionSchema with build: { watch: string | string[] }.
• Add devSessionWatchConfig to the theme spec that restricts watched paths to build.watch + locales/**.json + **.toml.

Motivation

Authors who use a separate source directory + build tool (common when writing Tailwind, TypeScript, or templated Liquid that compiles into blocks/) have no way to tell the CLI "only rebundle when the build is fully done." With this change, they can point the CLI at a sentinel file written last by their build tool:

name = "my-theme-extension"
type = "theme"
uid = "..."

[build]
watch = [".build-stamp"]

and get exactly one bundle cycle per build.

Changes

• packages/app/src/cli/models/extensions/specifications/theme.ts: extend schema with build.watch, add devSessionWatchConfig.
• Added a changeset (@shopify/app patch).

Test plan

• Theme app extension without [build] in its toml — dev watcher behaves exactly as before (watches whole extension dir).
• Theme app extension with [build] watch = [".build-stamp"] and an external build tool that touches .build-stamp last — shopify app dev only triggers one bundle cycle per build, instead of one per file write.
• Theme app extension with [build] watch = ["src/**"] — only changes under src/ trigger the watcher.
• locales/**.json and **.toml still trigger bundle cycles when build.watch is set (parity with function extensions).