Ruby: Error parsing embedded multiline blocks

**Description of the issue**

Seeing this error in the CodeQL run: `A parse error occurred. Check the syntax of the file. If the file is invalid, correct the error or exclude the file from analysis.`

The errored code block in question looks something like this and runs correctly.

```ruby
<<~BLOCKA
if some_condition?
<<~BLOCKB
BLOCKB
end
BLOCKA
```

Start by reproducing the CodeQL parsing error with the embedded Ruby example in the issue, focusing on nested multiline blocks. Trace the Ruby parser handling of the heredoc-like syntax; done means valid code with this structure parses without the reported error.

索引モデルが issue の本文から書いたものです。