chore: deprecate legacy Builder API

[bartlomieju]

Summary

• Adds @deprecated JSDoc tags to all public exports from @fresh/core/dev (Builder, BuildOptions, ResolvedBuildConfig, OnTransformArgs, OnTransformOptions, TransformFn)
• Adds @deprecated JSDoc tags to @fresh/plugin-tailwind and @fresh/plugin-tailwind-v3 exports
• Updates --builder flag in @fresh/init help text to show "(Deprecated)"
• Prints a warning when --builder flag is used during init

All deprecation notices point users to the Vite plugin as the replacement.

Closes #3635

Test plan

• Verify IDE shows deprecation strikethrough on Builder, BuildOptions, etc. when imported from fresh/dev
• Run deno run -A jsr:@fresh/init --builder . and verify deprecation warning is printed
• Type checks pass

🤖 Generated with Claude Code

[lunadogbot]

Scope is clean: JSDoc @deprecated on Builder, the five re-exported types, both Tailwind tailwind() entrypoints, and a runtime warning + help-text marker for --builder in @fresh/init. Lockfile untouched. Closes #3635. CI green; the failing deploy/deno/fresh preview is unrelated to a docstring/console-warn change and the sibling deploy/deno/denoland-fresh deploy is green.

One thing worth a follow-up: www/dev.ts:3 and packages/init/src/init.ts:559 still import { Builder } from "fresh/dev", so the docs site and the init scaffolder will themselves emit deprecation warnings in IDEs after this lands. The init.ts import is required by the --builder codegen path so that one can stay; www/ is the natural candidate to migrate next (or // deno-lint-ignore deprecation if migration is deferred).

• nit: the runtime warning at packages/init/src/init.ts:148 says "Use Vite instead (the default)" but doesn't point at the migration docs — a link would save users a search.
• nit: the test-plan items in the PR body are unchecked; worth running deno run -A jsr:@fresh/init --builder . once before merging to confirm the warning renders the way you want.