Make releaser tool update via a dir

[joaquimrocha]

This PR changes the releaser to use version based releases. It also updates the existing file based releases.

How to test

cd tools/releaser && npm run build

1. Test migration from legacy flat layout

This verifies that a plugin with a top-level artifacthub-pkg.yml gets migrated to a version directory.

# Pick a plugin that still has the old layout (e.g. app-catalog)
# Ensure it has a tarball first:
plugin-releaser package app-catalog

# Run with --no-commit so we can inspect the result
plugin-releaser artifacthub app-catalog --no-commit

Verify:

• app-catalog/<version>/artifacthub-pkg.yml exists with correct content
• app-catalog/<version>/README.md was copied in
• app-catalog/artifacthub-pkg.yml (top-level) was removed
• Console printed "Removed legacy top-level artifacthub-pkg.yml"

Clean up: git checkout -- app-catalog/

2. Test creating a new template

# Use a plugin that has no artifacthub-pkg.yml at all
plugin-releaser artifacthub <plugin-without-artifacthub> --template --no-commit

Verify:

• <plugin>/<version>/artifacthub-pkg.yml was created with TBD placeholders
• <plugin>/<version>/README.md was copied in
• No top-level artifacthub-pkg.yml was created

Clean up: rm -rf <plugin>/<version>/

3. Test updating an already-versioned plugin

This verifies it works correctly when the plugin already uses the new layout (like flux after our earlier commit).

# flux already has 0.5.0/ and 0.6.0/ directories
plugin-releaser artifacthub flux 0.6.0 --tarball flux/headlamp-k8s-flux-0.6.0.tar.gz --no-commit

Verify:

• flux/0.6.0/artifacthub-pkg.yml was updated (checksum/URL match the tarball)
• flux/0.6.0/README.md exists
• No top-level flux/artifacthub-pkg.yml was created

Clean up: git checkout -- flux/0.6.0/

4. Test duplicate template is rejected

plugin-releaser artifacthub flux --template --no-commit

Verify: exits with error "artifacthub-pkg.yml already exists for flux v0.6.0"