Add support for executing a script directly.

[ioquatix]

What was the end-user or developer problem that led to this PR?

I don't want to use any of the existing builders for compiling a native extension, because I want to use a build tool which is not in the list and I don't want to write a rakefile to wrap it either.

See the discussion here: #6403

What is your fix for the problem, implemented in this PR?

This enables spec.extensions = ["my_script.rb"]. That script will be executed with the appropriate environment variables.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[ioquatix]

The original issue was moved to a discussion: https://github.com/orgs/rubygems/discussions/6422

[ioquatix]

Another example usage: https://github.com/ruby/rdoc/pull/1113/files

[ioquatix]

@simi you closed the issue #6403, don't reply on the discussion #6422, what's the path forward here?

I've got a smaller to address a subset of this issue: #8879

But it still doesn't address the full scope of the problem.