Add return. We need a flag to say "I've returned, unwind everything" and a slot for the value. Add to Context:

returnFlag: bool = false,
returnValue: i64 = 0,

Handle return in executeStatement:

        if (stringsEqual(firstWord, "return")) {
            ctx.returnValue = parseExpression(ctx);
            if (ctx.source[ctx.pos] == ';') {
                ctx.pos += 1;
                skipSpaces(ctx.source, &ctx.pos);
            }
            ctx.returnFlag = true;
            return ctx.returnValue;
        }

Update executeBlock to bail out when the flag is set:

fn executeBlock(ctx: *Context) i64 {
    if (ctx.source[ctx.pos] == '{') {
        ctx.pos += 1;
        skipSpaces(ctx.source, &ctx.pos);
    }
    var last: i64 = 0;
    while (ctx.source[ctx.pos] != '}' and ctx.source[ctx.pos] != 0) {
        last = executeStatement(ctx);
        if (ctx.returnFlag) break;
    }
    if (ctx.source[ctx.pos] == '}') {
        ctx.pos += 1;
        skipSpaces(ctx.source, &ctx.pos);
    }
    return if (ctx.returnFlag) ctx.returnValue else last;
}

And the while handler: check ctx.returnFlag after each body execution, break out of the loop if it's set.