Reduce memory usage when reading a large script file.

[harawata]

Instead of reading the entire file into String, this version reads a file line by line.

This patch slightly changes the behavior for simplicity.

1. Version ≤ 3.3.1 allowed any character before @UNDO. 3.3.2 will allow only spaces and tabs. For example, the following undo marker was valid in ≤3.3.1, but will be invalid in 3.3.2.

   -- // some comment @UNDO
   

2. Version ≤3.3.1 removes the first double slash in any comment line. 3.3.2 will do that only in the @UNDO line.

   -- // some comment (original)
   -- some comment (≤3.3.1)
   -- // some comment (3.3.2)
   

Hi @h3adache ,
Is there any reason these changes could cause some problem?

[h3adache]

No I think those are good changes.
There might be people that put comments before the undo but that wasn't documented.
Our documentation says -- //@UNDO as if it was the token. This is already more lenient than that.

[harawata]

My thoughts exactly! Thank you, @h3adache !