if-else — minizig

Add if/else. When we see the word if, parse the condition in (), then execute or skip the { } block.

Two helpers:
- skip_block() -- jump past { ... } without executing, counting brace depth
- exec_block() -- execute statements inside { ... }

    check("if (5 > 3) { 10 } else { 20 }", 10);
    check("if (2 > 7) { 10 } else { 20 }", 20);
    check("var x: i64 = 10; if (x > 5) { x = 99; } x", 99);

fn skip_block() void {
    if (cur() == '{') { pos += 1; skip(); }
    var depth: i32 = 1;
    while (depth > 0 and cur() != 0) {
        if (cur() == '{') { depth += 1; }
        if (cur() == '}') { depth -= 1; }
        if (depth > 0) { pos += 1; }
    }
    if (cur() == '}') { pos += 1; skip(); }
}

fn exec_block() i64 {
    if (cur() == '{') { pos += 1; skip(); }
    var last: i64 = 0;
    while (cur() != '}' and cur() != 0) {
        last = exec_stmt();
        if (return_flag) { break; }
    }
    if (cur() == '}') { pos += 1; skip(); }
    return if (return_flag) return_val else last;
}

Add to exec_stmt, right after the var handler:

        if (streq(word, "if")) {
            if (cur() == '(') { pos += 1; skip(); }
            const cond: i64 = expression();
            if (cur() == ')') { pos += 1; skip(); }
            if (cond != 0) {
                const val: i64 = exec_block();
                if (is_letter(cur())) {
                    const s2: usize = pos;
                    const w2: []const u8 = read_name();
                    if (streq(w2, "else")) { skip_block(); } else { pos = s2; }
                }
                return val;
            } else {
                skip_block();
                if (is_letter(cur())) {
                    const s2: usize = pos;
                    const w2: []const u8 = read_name();
                    if (streq(w2, "else")) { return exec_block(); }
                    pos = s2;
                }
                return 0;
            }
        }