Append in Custom Syntax

[erikerikson]

What did you implement:

Respect custom variable syntaxes when appending to deep variables.

Closes #5769

How did you implement it:

How can we verify it:

See new unit test. For manual confirmation, run sls print against the following assets from #5769:

env.yml

description: this is dummy

serverless.yml - Errorneous template with a custom variable syntax

provider:
  name: aws
  runtime: nodejs8.10
  variableSyntax: "\\${{([ ~:a-zA-Z0-9._@\\'\",\\-\\/\\(\\)]+?)}}"

custom:
  settings: ${{file(./env.yml)}}

functions:
  hello:
    handler: handler.hello
    description: ${{self:custom.settings.description}}

Todos:

• Write tests
• [n/a] Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[pmuens]

Great stuff! Just tried it and it works fine 👍

Great job @erikerikson 💯

I haven't tested other variables usages thoroughly. Is there a huge likelyhood that this could break existing variables usages? The surface area of this change seems quite small and our Serverless Varaibles test coverage looks decent enough. Just asking since you and @eahefnawy are the Serverless Variables experts 😅

If that's not the case I'd say it should be GTM 👌

[erikerikson]

This is much more of a unhandled corner case that is now properly covered. 😄. I'd call the likelihood of breaking existing variable usage incredibly low but would be happy to have second opinions.

One of the big efforts I've made in the variables as part of my contribution is a large expansion of user level variable behavior specifications via tests. To be honest, mostly because I lost trust in my ability to reason over the effects of changes in the test could be complete and thorough, so every time there's a bug I've added coverage to make sure it doesn't crop back up. You can see the section around where I've added the test in this PR to see what I mean.