Stop blowing away the BufWritePre augroup in ftplugins

[johanventer]

Summary

The BufWritePre autocmds are reset each time a file type plugin is run, which means if you open a file of one type, it will auto format correctly, but if you then open a file of a different type, the original file will no longer auto format.

See further explanation here: #185 (comment)

This PR removes the augroup from each of the ftplugin files and adds some missing file extensions to the augroup in plugin/prettier.vim.

Test Plan

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[gadkadosh]

Didn't see this PR... I can confirm that a similar change was required for me to have vim-prettier auto format consistently and not break its own autocmds.