Allow configuration of mn2pdf stack and heap space

[ronaldtse]

From:

• https://github.com/metanorma/metanorma-cli/issues/326
• https://github.com/metanorma/iso-2533/issues/20

The change starts here: https://github.com/metanorma/mn2pdf-ruby/blob/7c981c12fac46b25c62845a965862433fd577391/lib/mn2pdf.rb#L8

We need to be able to change the JVM memory options:

• Xss stack size, defaults to 1m, we use 5m
• Xms heap initial size
• Xmx heap max size, defaults to half of physical memory up to 192m. We default to 2g. However for metanorma/iso-2533#20 a size of 6g was needed

So we need a way to configure these through library method invocation. These parameters are directly used by:

• https://github.com/metanorma/metanorma-cli/issues/326

[CAMOBAP]

I think it should read JAVA opts from env (JAVA_OPTS) and ensure that ss and mx are at least 5m and 2048m. this will allow to change it and give final client flexibility also for huge docs

An alternative approach for passing through metanorma-cli looks complex and redundant and make sense only for our code and not external tools

[CAMOBAP]

Implemented via JAVA_OPTS env var