SPICE-0028: Add support for multi-line string line continuations (#1507)

SPICE: https://github.com/apple/pkl-evolution/pull/31

pkl-formatter/src/test/files/FormatterSnippetTests/input/multi-line-strings.pkl

@@ -48,6 +48,26 @@ quux {
 		"""
 }
 
+// line continuations
+corge {
+
+  """
+  hello \
+  world
+  """
+
+#"""
+hello \#
+world
+"""#
+
+"""
+hello \
+\
+world
+"""
+}
+
 obj {
     data {
       ["bar"] = """

pkl-formatter/src/test/files/FormatterSnippetTests/output/multi-line-strings.pkl

@@ -52,6 +52,25 @@ quux {
   """
 }
 
+// line continuations
+corge {
+  """
+  hello \
+  world
+  """
+
+  #"""
+  hello \#
+  world
+  """#
+
+  """
+  hello \
+  \
+  world
+  """
+}
+
 obj {
   data {
     ["bar"] =