support specifying bun version

[haochenx]

We'd like to add an Options on Versions
bunjs-version (default: system) to allow specifying bun's version.
expected values are

• a specific version of bun, e.g. 1.0.30
• latest: will check and upgrade
• system: will use the whatever version already installed (if non is installed, nor bun bunx will be available)

and we intend to support arbitrary version by

• check if the already installed bunjs is of the specified version
  • if specified version is latest, bun upgrade --force will be executed at runtime
  • if specific version is specified and another version is installed,
    curl -fsSL https://bun.sh/install | (export export BUN_INSTALL=/usr/bin; bash -s -- bun-v${version})
    will be used to install the specified version