feat: adds else if and else conditional clauses

[claymcleod]

This change proposes extending conditional statements to include else if and else clauses.

These types of clauses have been proposed many times on Slack and more formally in #268 and #697. Overall, the feedback seems positive, though concerns have been raised as to whether the cost of added complexity is worth it (see #697 for more details). I have made my case at the end of #697 that I feel it's actually removing mental burden/complexity to add these clauses into the language.

TODO:

• Clarify that braces can be used in ternary expressions.
• Covers both expressions and workflow statements.
• Update logic to derive non-optional and optionals based on @cjllanwarne's comment.

Closes #268.

Before submitting this PR, please make sure:

• You have added a few sentences describing the PR here.
• You have added yourself or the appropriate individual as the assignee.
• You have updated the README.md or other documentation to account for these changes (when appropriate).
• You have updated the CHANGELOG.md describing the change and linking back to your pull request.
• You have read and agree to the CONTRIBUTING.md document.
• You have added or updated relevant example WDL tests to the specification.
  • See the guide for more details.

[a-frantz]

I feel it's actually removing mental burden/complexity to add these clauses into the language.

I agree with this, though I'll admit my programming background bias. I find it more complicated to create two unconnected if clauses with negated expressions than it would be to simply have an else (or else if) clause.

@geoffjentry said:

A question I'd have is: if one were to survey the world's breadth of public WDLs, how often do you see the above?

And I can point to at least one instance of this pattern in our workflows repository - https://github.com/stjudecloud/workflows/blob/main/workflows/qc/quality-check-standard.wdl#L351-L386

[claymcleod]

@peterhuene and @cjllanwarne, there still may be some polishing needed on the text, but let me know how this reads generally.