Add backtick string literals and formatter support

docs/ai_language_reference.md

@@ -15,15 +15,16 @@ Primary sources used: `lynglib/src/commonMain/kotlin/net/sergeych/lyng/{Parser,T
 
 ## 2. Lexical Syntax
 - Comments: `// line`, `/* block */`.
-- Strings: `"..."` (supports escapes). Multiline string content is normalized by indentation logic.
-  - Supported escapes: `\n`, `\r`, `\t`, `\"`, `\\`, `\uXXXX` (4 hex digits).
+- Strings: `"..."` or `` `...` `` (supports escapes). Multiline string content is normalized by indentation logic.
+  - Shared escapes: `\n`, `\r`, `\t`, `\\`, `\uXXXX` (4 hex digits).
+  - Delimiter escapes: `\"` inside `"..."`, ``\` `` inside `` `...` ``.
   - Unicode escapes use exactly 4 hex digits (for example: `"\u0416"` -> `Ж`).
   - Unknown `\x` escapes in strings are preserved literally as two characters (`\` and `x`).
   - String interpolation is supported:
-    - identifier form: `"$name"`
-    - expression form: `"${expr}"`
-    - escaped dollar: `"\$"` and `"$$"` both produce literal `$`.
-    - `\\$x` means backslash + interpolated `x`.
+    - identifier form: `"$name"` or `` `$name` ``
+    - expression form: `"${expr}"` or `` `${expr}` ``
+    - escaped dollar: `"\$"`, `"$$"`, `` `\$` ``, and `` `$$` `` all produce literal `$`.
+    - `\\$x` means backslash + interpolated `x` in either delimiter form.
   - Per-file opt-out is supported via leading comment directive:
     - `// feature: interpolation: off`
     - with this directive, `$...` stays literal text.

docs/tutorial.md

@@ -1654,15 +1654,27 @@ The type for the character objects is `Char`.
 
 ### String literal escapes
 
+Lyng string literals can use either double quotes or backticks:
+
+    val a = "hello"
+    val b = `hello`
+    assert(a == b)
+
 | escape | ASCII value           |
 |--------|-----------------------|
 | \n     | 0x10, newline         |
 | \r     | 0x13, carriage return |
 | \t     | 0x07, tabulation      |
 | \\     | \ slash character     |
-| \"     | " double quote        |
 | \uXXXX | unicode code point    |
 
+Delimiter-specific escapes:
+
+| form   | escape | value            |
+|--------|--------|------------------|
+| `"..."` | \"    | " double quote   |
+| `` `...` `` | \` | ` backtick       |
+
 Unicode escape form is exactly 4 hex digits, e.g. `"\u263A"` -> `☺`.
 
 Other `\c` combinations, where c is any char except mentioned above, are left intact, e.g.:
@@ -1695,10 +1707,15 @@ Example:
 
     val name = "Lyng"
     assertEquals("hello, Lyng!", "hello, $name!")
+    assertEquals("hello, Lyng!", `hello, $name!`)
     assertEquals("sum=3", "sum=${1+2}")
+    assertEquals("sum=3", `sum=${1+2}`)
     assertEquals("\$name", "\$name")
     assertEquals("\$name", "$$name")
+    assertEquals("\$name", `\$name`)
+    assertEquals("\$name", `$$name`)
     assertEquals("\\Lyng", "\\$name")
+    assertEquals("\\Lyng", `\\$name`)
     >>> void
 
 Interpolation and `printf`-style formatting can be combined when needed: