Fix escaping in yaml strings (#1165)

The backslash needs to be escaped when rendering double-quoted YAML strings.

In addition, this escapes the following characters:

* next line (0x85)
* nbsp (0xa0)

pkl-core/src/test/files/LanguageSnippetTests/input/api/yamlRenderer9.yml.pkl

@@ -0,0 +1,36 @@
+// test escaping in double quotes
+// every string is prefixed with `\t` s.t. YamlRenderer will emit double-quoted strings.
+
+`null` = "\t\u{00}"
+
+bell = "\t\u{7}"
+
+backspace = "\t\u{8}"
+
+horizontalTab = "\t"
+
+lineFeed = "\t\u{a}"
+
+verticalTab = "\t\u{b}"
+
+formFeed = "\t\u{c}"
+
+carriageReturn = "\t\r"
+
+escape = "\t\u{1b}"
+
+doubleQuote = "\t\""
+
+backslash = "\t\\"
+
+nextLine = "\t\u{85}"
+
+nbsp = "\t\u{a0}"
+
+lineSep = "\t\u{2028}"
+
+paragraphSep = "\t\u{2029}"
+
+output {
+  renderer = new YamlRenderer {}
+}

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

@@ -114,12 +114,12 @@ examples {
     render(".NAN")
     render(".nAn")  // never float
   }
-  
+
   ["tag like strings"] {
     "!!bool true"
     "!!str my string value"
   }
-  
+
   ["number like string keys"] {
     render(new Dynamic {
       `0` = "0"

pkl-core/src/test/files/LanguageSnippetTests/output/api/yamlRenderer9.yml

@@ -0,0 +1,15 @@
+'null': "\t\0"
+bell: "\t\a"
+backspace: "\t\b"
+horizontalTab: "\t"
+lineFeed: "\t\n"
+verticalTab: "\t\v"
+formFeed: "\t\f"
+carriageReturn: "\t\r"
+escape: "\t\e"
+doubleQuote: "\t\""
+backslash: "\t\\"
+nextLine: "\t\N"
+nbsp: "\t\_"
+lineSep: "\t\L"
+paragraphSep: "\t\P"