fix(vector-code-nvim): install or upgrade vectorcode-cli based on cli existence

[cristobalgvera]

📑 Description

This pull request updates the build logic for the VectorCode plugin to improve how dependencies are checked and managed. The main change is that it now conditionally installs or upgrades the vectorcode tool based on its presence, and uses vim.system for better process handling.

Dependency management improvements:

• The build function now checks if uv is installed by verifying that vim.fn.executable "uv" returns 0, and throws an error if not.
• The script now determines whether to install or upgrade vectorcode by checking if it is already executable, and then calls uv tool install vectorcode or uv tool upgrade vectorcode accordingly using vim.system for process execution.

[github-actions]

Review Checklist

Does this PR follow the [Contribution Guidelines](development guidelines)? Following is a partial checklist:

Proper conventional commit scoping:

• If you are adding a new plugin, the scope would be the name of the category it is being added into. ex. feat(utility): added noice.nvim plugin

• If you are modifying a pre-existing plugin or pack, the scope would be the name of the plugin folder. ex. fix(noice-nvim): fix LSP handler error

• Pull request title has the appropriate conventional commit type and scope where the scope is the name of the pre-existing directory in the project as described above

• README is properly formatted and uses fenced in links with <url> unless they are inside a [title](url)

• Entry returns a single plugin spec with the new plugin as the only top level spec (not applicable for recipes or packs).

• Proper usage of opts table rather than setting things up with the config function.

• Proper usage of specs table for all specs that are not dependencies of a given plugin (not applicable for recipes or packs).

[Uzaaft]

Sorry for the delay in reviewing this.

[Uzaaft]

we should support uv and pipx.
If uv is installed, use uv no matter what. fallback to pipx, and none of them are there, notify the end-user. Also update the docs to reflect this.