Swap from pkg to npm sea for jbrowse-cli

[hextraza]

• Replaces the jb-pkg task in package.json and build.gradle with a jb-sea task that executes two .mjs scripts (which should work cross-platform)
• fetch-jbrowse-bundle.mjs downloads the latest jbrowse cli and bundles it into a single file so that node can SEA it
• build-sea.mjs outputs a SEA jbrowse cli for the current platform by using postject and node's --experimental-sea-config mode. This requires >node 20 and some method of executing postject (npm exec, npx, etc). It also attempts to deal with executable signing and chmod permissions. The executable should match the names we already had for files in the ./resources/external/jb-cli folder