feat: string escape sequences (#215)

Author: novusnota

pages/book/expressions.mdx

@@ -68,17 +68,69 @@ Read more about booleans and [`Bool{:tact}`](/book/types#booleans) type in the d
 A string literal is zero or more characters enclosed in double (`"`) quotation marks. All string literals are objects of [`String{:tact}`][p] type.
 
 ```tact
-"foo"
-"1234"
+"foo";
+"1234";
+```
+
+Tact strings support a range of [escape sequences](https://en.wikipedia.org/wiki/Escape_sequence) starting with a backslash `\\` character:
+
+* `\\{:tact}` — literal backslash
+* `\"{:tact}` — double quote
+* `\n{:tact}` — newline
+* `\r{:tact}` — carriage return
+* `\t{:tact}` — tab
+* `\v{:tact}` — vertical tab
+* `\b{:tact}` — backspace
+* `\f{:tact}` — form feed
+* `\x00{:tact}` through `\xFF{:tact}` — [code point](https://en.wikipedia.org/wiki/Code_point), must be exactly two hex digits long
+* `\u0000{:tact}` through `\uFFFF{:tact}` — [Unicode code point][unicode], must be exactly four hex digits long
+* `\u{0}{:tact}` through `\u{FFFFFF}{:tact}` — [Unicode code point][unicode], can be from $1$ to $6$ hex digits long
+
+[unicode]: https://en.wikipedia.org/wiki/Unicode#Codespace_and_code_points
+
+```tact
+// \\
+"escape \\ if \\ you \\ can \\";
+
+// \"
+"this \"literally\" works";
+
+// \n
+"line \n another line";
+
+// \r
+"Shutters \r Like \r This \r One";
 
-// Note, that at the moment Tact strings can't have escape characters in them:
-"line \n another"; // SYNTAX ERROR!, see: https://github.com/tact-lang/tact/issues/25
+// \t
+"spacing \t granted!";
 
-// This means, that double quotes inside strings are also prohibited:
-"this \"can't be!\""; // SYNTAX ERROR!
+// \v
+"those \v words \v are \v aligned";
+
+// \b
+"rm\b\bcreate!";
+
+// \f
+"form \f feed";
+
+// \x00 - \xFF
+"this \x22literally\x22 works"; // \x22 represents a double quote
+
+// \u0000 - \uFFFF
+"danger, \u26A1 high voltage \u26A1"; // \u26A1 represents the ⚡ emoji
+
+// \u{0} - \u{FFFFFF}
+"\u{1F602} LOL \u{1F602}"; // \u{1F602} represents the 😂 emoji
 ```
 
-Read more about strings and [`String{:tact}`][p] type there: [Primitive types][p].
+<Callout>
+
+  Read more about strings and [`String{:tact}`][p] type:\
+  [Primitive types in the Book][p]\
+  [Strings and StringBuilders in the Reference](/ref/api-strings)
+
+</Callout>
+
 
 ### `null` literal