fix!: rename the _fresh build folder to .fresh (BREAKING)

[csvn]

As mentioned in a prior discussion, it seemed like renaming _fresh build folder to .fresh would align more with generated files than the current _. This PR implements that suggestion, to make the breaking change potentially sooner rather than later.

Prior arts

• node_modules/.deno - Deno installed packages
• .vite - Cache of prebuilt packages and files
• .angular - Cache of built files and metadata
• .github/ or .vscode/ folders

The . prefix in general seems to be used either for generated files, files that should be considered "hidden", or configuration files (e.g. .editorconfig or .gitignore). This seems to fit well with outDir, and would make naming more consistent IMO.

[fry69]

I strongly object to this. .fresh is a hidden folder, those usually reserved for configuration files or ephemeral caches.

Generated output should be visible in a clearly marked/named folder. Common folder names are:

• dist
• out
• build
• public (that may clash with static a bit)

The examples also are not fitting IMHO:

• node_modules/.deno` is a special case, it's wild west inside that hierarchy
• .vite/.angular are cache/staging folders, not generated output. Cache/staging folders are ephemeral, those can be deleted without any side effect, unlike generated output folders, which might break a running site
• .github/.vscode are well-known special purpose configuration folders

If no agreement on a new folder name for the generated output can be reached, please keep the _fresh name, it is at least not a hidden folder.

[fibibot]

The rename touches deno.json, .gitignore, packages/init, the deploy workflow, docs, and the migration tool consistently. One thing worth confirming intent on: the asset URL prefix is still _fresh — packages/fresh/src/dev/builder.ts:333 (/_fresh/js/${BUILD_ID}/), packages/fresh/src/dev/dev_build_cache.ts:581, and packages/fresh/src/middlewares/static_files.ts:87. After this PR the on-disk folder is .fresh but the URL path served to the browser is still _fresh/js/.... packages/fresh/tests/islands_test.tsx:801 still asserts the _fresh/js/ URL too. Intentional (URL path independent of build folder), or should they move together?

• nit: packages/update/src/update.ts:140 only matches tasks.preview === "deno serve -A _fresh/server.js" as an exact string — a user who customised the preview task (e.g. added --port) won't be migrated.
• nit: the migration tool doesn't touch the user project's .gitignore, so existing projects will need a manual _fresh/ → .fresh/ swap there.

CI green on b24d008.