Add on directive to switch as an alternative to case

[scrhartley]

This is a copy of #106 which was backed out after a problem with the contributors agreement.

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The current switch directive is not recommended due to its fall-through behavior being regarded as error-prone. To solve this problem, we introduce a new on directive as an alternative to case, which doesn't support fall-through. This new directive allows specifying multiple comma-separated conditions in order to address the primary motivator for using fall-through with case.

Example

Using case:

<#switch animal.size>
  <#case "tiny">
  <#case "small">
     This will be processed if it is small
     <#break>
  <#case "medium">
     This will be processed if it is medium
     <#break>
  <#default>
     This will be processed if it is neither
</#switch>

Using on:

<#switch animal.size>
  <#on "tiny", "small">
     This will be processed if it is small
  <#on "medium">
     This will be processed if it is medium
  <#default>
     This will be processed if it is neither
</#switch>

Details

• Mixing case and on directives is disallowed and will fail.
• If a switch contains an on, then using break or continue is not supported (applying to both on and default). In this situation, break and continue will follow the behavior of the containing scope, e.g. of a containing list. This does mean that if someone accidentally uses break with on, they could potentially become confused.

Additional Notes

• When on is not used, the legacy behavior for case/default is that continue will be treated as break. If we wish to change this, then that work can be done separately.
• If using default, the parser also allows it to appear before or between case directives, but when using on then default is only allowed after.

[ddekany]

Thanks, was merged. Also I have applied exactly the same changes as after PR #106, just have put all 3 commits into 1 here.