Lua: support extensions in custom writers

Custom writers can define the extensions that they support via the
global `writer_extensions`. The variable's value must be a table with
all supported extensions as keys, and their default status as values.

E.g., the below specifies that the writer support the extensions `smart`
and `sourcepos`, but only the `smart` extension is enabled by default:

    writer_extensions = {
      smart = true,
      sourcepos = false,
    }

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