Additional test cases for slashdash and multi-line strings

tests/test_cases/expected_kdl/multiline_string_whitespace_only.kdl

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+node "" "" "   \n"

tests/test_cases/input/multiline_string_whitespace_only.kdl

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+// This file deliberately contains unusual whitespace
+// the first two strings are empty
+node """
+  	""" """
+ 	 \
+             
+ 	 """ \
+    \ // The next string contains only whitespace
+    """
+       
+
+    """

tests/test_cases/input/slashdash_after_type_fail.kdl

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+node (type) /- arg1 arg2

tests/test_cases/input/slashdash_between_child_blocks_fail.kdl

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+node { one } /- { two } { three }

tests/test_cases/input/slashdash_in_prop_fail.kdl

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+node key /- = value