Add typed JSON output to theme preview

[gonzaloriestra]

WHY are these changes introduced?

theme preview already emits JSON, but its result has no discoverable schema and its service also renders output and opens the browser.

Part of https://github.com/shop/issues-develop/issues/23695.

WHAT is this pull request doing?

Return a typed preview result and move presentation and browser opening into the command. Preserve the compact JSON keys, success messages, file/API errors, and nonfatal browser behavior. Browser failures use typed diagnostics in JSON mode.

Also, the JSON output was previously using stderr, but it was fixed to use stdout. Warnings and other diagnostics stay on stderr.

Normal output:

Preview is ready
• https://abc123.shopifypreview.com
• Preview ID: abc123

Matching result:

{"url":"https://abc123.shopifypreview.com","preview_identifier":"abc123"}

How to manually test your changes?

pnpm shopify theme preview --store YOUR_STORE --theme YOUR_THEME --overrides overrides.json
pnpm shopify theme preview --store YOUR_STORE --theme YOUR_THEME --overrides overrides.json --preview-id YOUR_PREVIEW --open
pnpm shopify theme preview --store YOUR_STORE --theme YOUR_THEME --overrides overrides.json --json
pnpm shopify theme preview --help
pnpm shopify theme preview --json-schema

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• A single changeset for all theme migrations is added in the last PR, #8682.

[github-actions]

Differences in type declarations

We detected differences in the type declarations generated by Typescript for this branch compared to the baseline ('main' branch). Please, review them to ensure they are backward-compatible. Here are some important things to keep in mind:

• Some seemingly private modules might be re-exported through public modules.
• If the branch is behind main you might see odd diffs, rebase main into this branch.

New type declarations

We found no new type declarations in this PR

Existing type declarations

packages/cli-kit/dist/public/node/base-command.d.ts

@@ -35,6 +35,7 @@ declare abstract class BaseCommand extends Command {
         argv: string[];
     }>;
     protected environmentsFilename(): string | undefined;
+    protected validateNonTTYFlags(flags: FlagOutput): void;
     protected failMissingNonTTYFlags(flags: FlagOutput, requiredFlags: string[]): void;
     private failMissingNonTTYFlagRequirements;
     private applicableNonTTYFlagRequirements;