Provide `:BuildComposer` command

[maujim]

nvim-treesitter has a nice installation method using vim-plug that doesn't require users to add a custom function:
Plug 'nvim-treesitter/nvim-treesitter', {'do': ':TSUpdate'}

More info: https://github.com/nvim-treesitter/nvim-treesitter#installation

If the BuildComposer function from the readme can be wrapper in a command and shipped with the plugin, that would be nice. Would make the installation only require a single line (for vim-plug, which is already the recommended method)

This may not be possible in pure vimscript since treesitter uses lua to expose this command