feat: syntax for using first available formatter

Alternations are now supported. You can specify a sub-list in place of a
formatter name and conform will use the first formatter in that list
that is available. For example, this will use either prettierd or
prettier (whichever is available), and then always trim whitespace
afterwards:

conform.format(formatters = { { "prettierd", "prettier" }, "trim_whitespace" })

This syntax is available both in the formatters_by_ft config option and
in the `formatters` argument of the `format` method.

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