Fix multi-line-string-body grammar (#552)

* Make multi-line-string-body non-greedy

* Don't require string-character after quotes in multiline strings

* Prevent accidentally consuming triple quotes in multi-line-string-body

* Add test cases

draft-marchan-kdl2.md

@@ -991,7 +991,7 @@ quoted-string :=
     (multi-line-string-body newline)?
     (unicode-space | ws-escape)* '"""'
 single-line-string-body := (string-character - newline)*
-multi-line-string-body := (('"' | '""')? string-character)*
+multi-line-string-body := ('"' ^'"' | '""' ^'"' | string-character)*?
 string-character :=
     '\\' (["\\bfnrts] |
     'u{' hex-unicode '}') |

tests/test_cases/expected_kdl/multiline_string_only_1_quote.kdl

@@ -0,0 +1 @@
+node "\""

tests/test_cases/expected_kdl/multiline_string_only_2_quotes.kdl

@@ -0,0 +1 @@
+node "\"\""

tests/test_cases/input/multiline_string_only_1_quote.kdl

@@ -0,0 +1,3 @@
+node """
+"
+"""

tests/test_cases/input/multiline_string_only_2_quotes.kdl

@@ -0,0 +1,3 @@
+node """
+""
+"""