Support FLAGS_COALESCE on AST_ASSIGN_OP (`??=`)

Add AST_PROP_GROUP in version 70.

The type information is deliberately left out of versions < 70.

`children['type']` is a Node describing the type of the property group,
or null

Add AST_PROP_GROUP in version 70.

The type information is deliberately left out of versions < 70.

`children['type']` is a Node describing the type of the property group,
or null.
`children['props']` is the AST_PROP_DECL.

The property visibility modifier flags were moved to the AST_PROP_GROUP.

Update package.xml

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Remove unused AST kinds

Add explicit ARRAY_ELEM_REF flag

Instead of using magic value "1". Add a test for by-ref destructing
functionality in PHP 7.3. It also worked previously, this is just
improving the d

Add explicit ARRAY_ELEM_REF flag

Instead of using magic value "1". Add a test for by-ref destructing
functionality in PHP 7.3. It also worked previously, this is just
improving the dumping functionality.

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Use ast\Metadata object for ast\get_metadata()

Make flags information slightly more accurate

Add ast\get_metadata() function

This function returns metadata about the AST kinds. Currently this
mainly provides the flags supported by each AST kind.