add pkg.pr.new CI workflow for preview releases

[remorses]

Closes #449

Adds a GitHub Actions workflow that publishes preview packages on every push and PR using pkg.pr.new.

This enables users to test changes from any commit/PR without waiting for a release:

npm i https://pkg.pr.new/@opentui/core@<commit>

Publishes all 4 packages: core, react, solid, vue.

Example: See remorses/opentui#5 for a working example of how pkg.pr.new comments look on PRs.

Note: Requires the pkg.pr.new GitHub App to be installed on this repo (this is why the "Publish Preview" check is currently failing).

[Copilot]

Pull request overview

This PR adds a GitHub Actions workflow that automatically publishes preview packages using pkg.pr.new on every push and pull request. This enables users and contributors to test changes from any commit or PR without waiting for an official release by installing packages via URLs like npm i https://pkg.pr.new/@opentui/core@<commit>.

Key Changes:

• Automated preview package publishing for all four packages (core, react, solid, vue) on every push and PR
• Build pipeline that compiles all packages with appropriate flags before publishing
• Uses pkg.pr.new service to host temporary package previews

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[msmps]

I think it's safe to remove vue from the workflow as it's no longer maintained (or released) and will be removed from this repo (#236 (comment))

[remorses]

I removed vue. Someone with enough permissions need to install the GitHub app now.

[remorses]

This PR would make it much easier for opencode forks to try out new PRs features and find if there are any issues before merging. Can someone take a look?

[pkg-pr-new]

npm i https://pkg.pr.new/anomalyco/opentui/@opentui/core@455

npm i https://pkg.pr.new/anomalyco/opentui/@opentui/react@455

npm i https://pkg.pr.new/anomalyco/opentui/@opentui/solid@455

commit: ef1c350