Assemble NodeDumper result in property

Append to a property instead of returning strings. This is
significantly faster when dumping large ASTs.

This also fixes an inconsistency

Assemble NodeDumper result in property

Append to a property instead of returning strings. This is
significantly faster when dumping large ASTs.

This also fixes an inconsistency where the indentation level for
strings and comments was off-by-one.

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Update phpstan baseline

Fix PhpStan errors

Declare list types (#907)

Closes #905

Use PhpStan level 6

New baseline errors are array types that I prefer to leave alone,
as well as one PhpStan bug:
https://github.com/phpstan/phpstan/issues/4526

Use PHPStan level 5

Add phpstan baseline