Fix errors around strings with lone high or low surrogates (#1673)

This fixes some errors around treatment of strings with lone high/low
surrogates.

pkl-core/src/test/files/LanguageSnippetTests/input/api/string.pkl

@@ -415,6 +415,7 @@ examples {
     str1.substring(2, 3)
     str1.substring(2, 4)
     str1.substring(0, 7)
+    "\u{D800}hello".substring(0, 2)
     module.catch(() -> str1.substring(-1, 4))
     module.catch(() -> str1.substring(1, 8))
     module.catch(() -> str1.substring(3, 2))
@@ -436,6 +437,15 @@ examples {
     str1.getOrNull(3)
     str1.getOrNull(6)
     str1.getOrNull(7)
+    "🏀".getOrNull(0)
+    "🏀".getOrNull(1)
+    "\u{D800}".getOrNull(0)
+    "\u{D800}".getOrNull(1)
+    "\u{D800}h".getOrNull(0)
+    "\u{D800}h".getOrNull(1)
+    "\u{D800}h".getOrNull(2)
+    "h\u{D800}".getOrNull(0)
+    "h\u{D800}".getOrNull(1)
   }
 
   ["toCodePoints()"] {

pkl-core/src/test/files/LanguageSnippetTests/input/basic/string.pkl

@@ -59,6 +59,13 @@ examples {
     str3[2]
     module.catch(() -> str3[-1])
     module.catch(() -> str3[4])
+
+    "🏀"[0]
+    "\u{D800}"[0]
+    "\u{D800}h"[0]
+    "\u{D800}h"[1]
+    "h\u{D800}"[0]
+    "h\u{D800}"[1]
   }
 
   ["dollar sign has no special meaning"] {

pkl-core/src/test/files/LanguageSnippetTests/output/api/string.pcf

@@ -353,6 +353,7 @@ examples {
     "c"
     "cd"
     "abcdefg"
+    "?h"
     "Character index `-1` is out of range `0`..`7`. String: \"abcdefg\""
     "Character index `8` is out of range `1`..`7`. String: \"abcdefg\""
     "Character index `2` is out of range `3`..`7`. String: \"abcdefg\""
@@ -372,6 +373,15 @@ examples {
     "d"
     "g"
     null
+    "🏀"
+    null
+    "?"
+    null
+    "?"
+    "h"
+    null
+    "h"
+    "?"
   }
   ["toCodePoints()"] {
     List(97, 98, 99, 100, 101, 102, 103)

pkl-core/src/test/files/LanguageSnippetTests/output/basic/string.pcf

@@ -51,6 +51,12 @@ examples {
     "i"
     "Character index `-1` is out of range `0`..`3`. String: \"this\""
     "Character index `4` is out of range `0`..`3`. String: \"this\""
+    "🏀"
+    "?"
+    "?"
+    "h"
+    "h"
+    "?"
   }
   ["dollar sign has no special meaning"] {
     "123$"