js: fix codegen for multiline strings (#6791)

vlib/v/util/quote.v

@@ -0,0 +1,97 @@
+module util
+
+import strings
+
+const (
+	invalid_escapes = ['(', '{', '$', '`', '.']
+)
+
+pub fn smart_quote(str string, raw bool) string {
+	len := str.len
+	if len == 0 {
+		return str
+	}
+	mut result := strings.new_builder(0)
+	mut pos := -1
+	mut last := ''
+	// TODO: This should be a single char?
+	mut next := ''
+	mut skip_next := false
+	for {
+		pos = pos + 1
+		if skip_next {
+			skip_next = false
+			pos = pos + 1
+		}
+		if pos >= len {
+			break
+		}
+		if pos + 1 < len {
+			unsafe {
+				next = str.str[pos + 1].str()
+			}
+		}
+		mut current := str
+		mut toadd := str
+		if len > 1 {
+			unsafe {
+				current = str.str[pos].str()
+			}
+			toadd = current
+		}
+		// double quote
+		if current == '"' {
+			toadd = '\\"'
+			current = ''
+		}
+		if current == '\\' {
+			if raw {
+				toadd = '\\\\'
+			} else {
+				// escaped backslash - keep as is
+				if next == '\\' {
+					toadd = '\\\\'
+					skip_next = true
+				} else if next != '' {
+					if raw {
+						toadd = '\\\\' + next
+						skip_next = true
+					}
+					// keep all valid escape sequences
+					else if next !in invalid_escapes {
+						toadd = '\\' + next
+						skip_next = true
+					} else {
+						toadd = next
+						skip_next = true
+					}
+				}
+			}
+		}
+		// keep newlines in string
+		if current == '\n' {
+			toadd = '\\n'
+			current = ''
+		} else if current == '\r' && next == '\n' {
+			toadd = '\r\n'
+			current = ''
+			skip_next = true
+		}
+		// Dolar sign
+		if !raw && current == '$' {
+			if last == '\\' {
+				toadd = r'\$'
+			}
+		}
+		// Windows style new line \r\n
+		if !raw && current == '\r' {
+			if next == '\n' {
+				skip_next = true
+				toadd = '\\n'
+			}
+		}
+		result.write(toadd)
+		last = current
+	}
+	return result.str()
+}