Add Lyng string interpolation with docs/tests and refresh changelog

docs/ai_language_reference.md

@@ -17,6 +17,14 @@ Primary sources used: `lynglib/src/commonMain/kotlin/net/sergeych/lyng/{Parser,T
   - Supported escapes: `\n`, `\r`, `\t`, `\"`, `\\`, `\uXXXX` (4 hex digits).
   - 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`.
+  - Per-file opt-out is supported via leading comment directive:
+    - `// feature: interpolation: off`
+    - with this directive, `$...` stays literal text.
 - Numbers: `Int` (`123`, `1_000`), `Real` (`1.2`, `1e3`), hex (`0xFF`).
 - Char: `'a'`, escaped chars supported.
   - Supported escapes: `\n`, `\r`, `\t`, `\'`, `\\`, `\uXXXX` (4 hex digits).

docs/tutorial.md

@@ -1573,6 +1573,52 @@ same as:
     assert(s[1] == 'a')
     >>> void
 
+### String interpolation
+
+Supported forms:
+
+- `$name`
+- `${expr}`
+
+Literal dollar forms:
+
+- `\$` -> `$`
+- `$$` -> `$`
+
+Example:
+
+    val name = "Lyng"
+    assertEquals("hello, Lyng!", "hello, $name!")
+    assertEquals("sum=3", "sum=${1+2}")
+    assertEquals("\$name", "\$name")
+    assertEquals("\$name", "$$name")
+    assertEquals("\\Lyng", "\\$name")
+    >>> void
+
+Interpolation and `printf`-style formatting can be combined when needed:
+
+    val method = "transfer"
+    val argc = 2
+    val compact = "%s:%d"(method, argc)
+    assertEquals("call=transfer:2", "call=$compact")
+    assertEquals("[transfer:2] ok", "[${"%s:%d"(method, argc)}] ok")
+    >>> void
+
+Interpolation also works well with regex patterns. To keep a literal `$` in the
+regex, escape it in the resulting pattern:
+
+    val currency = "USD"
+    val amount = 15
+    val escapedDollar = "\\$"
+    val re = Regex("^${currency}${escapedDollar}${amount}$")
+    assert("USD$15" =~ re)
+    assert("USD15" !~ re)
+    >>> void
+
+If you need old literal behavior in a file, add a leading directive comment:
+
+    // feature: interpolation: off
+
 ### Char literal escapes
 
 Are the same as in string literals with little difference: