Fix handling multiple levels of nested blocks

[delichik]

When parsing something like:

foobar1 {
    sub1 arg1
    sub2
    sub_foobar1 sub_foobar1_arg {
        sub3 arg3
        sub4
    }
}

foobar2 {
}

I noticed a strange issue:

• Calling Next() on sub_foobar1 works fine — I correctly get sub_foobar1_arg.
• But right after that, calling NextBlock() and then Val() gives me… {.

What I actually expect there is sub3.

Changes:

When nesting > 0, if the next token is {, it means we've entered a new block.
So we move the cursor back and continue with the same logic as when nesting == 0 in old code.

Now NextBlock() correctly steps into the nested block and Val() returns the expected first item (sub3).