Allow blocks in interpolated string expressions

[HABER7789]

Allow blocks in interpolated string expressions

Previously the raw lexer tracked interpolation depth with a single u8 counter, which only balanced one level of braces and couldn't handle a { ... } block or a nested interpolation inside ${ ... }. As a result, expressions such as a for loop or a block inside an interpolated string failed to parse.

This change replaces that counter with a Vec<u32> stack that tracks brace depth per interpolation level: { increments the innermost level, } at depth greater than zero closes a block brace, and } at depth zero ends the interpolation. Blocks and nested interpolations now balance correctly, and the old 255-level u8 overflow panic risk is gone.

I also added lexer, parser, and evaluator tests covering blocks, nested blocks, for loops, and an unclosed block.

This PR Fixes #3463.

Example:

function Main() : String {
    let x = 5;
    $"result is { { let y = x + 1; y * 2 } }"   // -> "result is 12"
}