Introduce strings template param to force add file name enum

[viktorasl]

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During migration my team and myself moved some files which used L10n structure to other frameworks, after moving .strings file there as well we are not be able to use the same L10n.<FileName> structure as there is only a single .strings file there.

As a solution I've introduced a new file template, but the change is simple, so to keep consistency in those kind of situations I'd like to suggest an option to force add file name enums.

• Documentation updated
• Not a breaking change, option not being provided defaults to old behaviour
• Updated tests
• Not added support for swift3

Cheers! ☮️

[SwiftGen-Eve]

Hey 👋 I'm Eve, the friendly bot watching over SwiftGen 🤖

Thanks a lot for your contribution!

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Seems like everything is in order 👍 You did a good job here! 🤝

Generated by 🚫 Danger

[AliSoftware]

Before merging this PR we'll need to:

• Merge Obsolete swift3 templates #691 (obsolete Swift 3 templates)
• Rebase this PR on top of develop (this should also fix the CI failure currently present in this PR due to a change in our CI config recently merged)
• Update this PR to remove the special cases that have been introduced for Swift 3 (rightfully at that time, but they won't be necessary once we drop Swift 3 after Obsolete swift3 templates #691)

[AliSoftware]

Thanks again @viktorasl for the PR! Merging now.

Our next step is to create another PR to apply that same change to other templates (an not just for strings template) 👍