added \' escape character

compiler/src/prog8/ast/antlr/Antr2Kotlin.kt

@@ -675,6 +675,7 @@ internal fun unescape(str: String, position: Position): String {
                 'n' -> '\n'
                 'r' -> '\r'
                 '"' -> '"'
+                '\'' -> '\''
                 'u' -> {
                     "${iter.nextChar()}${iter.nextChar()}${iter.nextChar()}${iter.nextChar()}".toInt(16).toChar()
                 }

docs/source/syntaxreference.rst

@@ -415,6 +415,7 @@ There are several escape sequences available to put special characters into your
 - ``\n`` - newline character (move cursor down and to beginning of next line)
 - ``\r`` - carriage return character (more or less the same as newline if printing to the screen)
 - ``\"`` - quote character (otherwise it would terminate the string)
+- ``\'`` - apostrophe character (has to be escaped in character literals, is okay inside a string)
 - ``\uHHHH`` - a unicode codepoint \u0000 - \uffff (16-bit hexadecimal)
 - ``\xHH`` - 8-bit hex value that will be copied verbatim *without encoding*