(entry_body): directly parse entry_body_formal_part
In Ada grammar the entry_body_formal_part both the
entry_index_specification and the parameter_profile are optional
(because they describe both simple entries and entry families,
both with and without parameters). However, tree-sitter does not
allow rules that match empty strings.
The simplest solution is to directly encode entry_body_formal_part
within the entry_body.
add test.
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