[FIX JENKINS-44390] Support reading nested stages within parallel, generate new syntax

[kzantow]

Description

Use some new-style nested parallel stages - e.g. Cmd/Ctrl-S and paste examples in, see what the editor generates. Validate it is using the new style nested stage parallel syntax.

See JENKINS-44390.

Submitter checklist

• Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description
• Appropriate unit or acceptance tests or explanation to why this change has no tests
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verified the change matches the issue description
• Reviewed the code
• Verified that the appropriate tests have been written or valid explanation given

Esp: @abayer @michaelneale

[michaelneale]

nice, taking a look. Cool that it touches so few files.

[kzantow]

Closing as superseded by #1375, which includes these changes