Fix XEP-0388 continue child declaration order

[rallep71]

The XEP-0388 schema defines the children of continue in this order:

1. additional-data
2. tasks
3. text

xmpp_codec.spec currently declares additional-data, text, tasks. This
patch moves tasks before text so that the source specification matches the
schema.

This is separate from the reference-group ordering problem in fast_xml,
which is addressed by processone/fast_xml#53. Both corrections are
needed for the generated XEP-0388 codec to emit the expected order.

I have not edited the generated codec. The repository's contribution guidance
asks contributors not to commit generated files, and the full regeneration
should in any case use the fast_xml change accepted upstream.

Related to processone/ejabberd#4606.

Testing

I tested this with the pinned xmpp 1.13.4 and fast_xml 1.1.60 sources. The stock
codec serialises continue as text, tasks, additional-data. With the
generator and specification fixes applied, it serialises as
additional-data, tasks, text.

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[p1bot]

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