feat: Conversion from fixed-form to free-form using `findent`

[kc9jud]

Is there an existing request for this?

• [X] I have searched the existing issues

Feature Request

findent has an option to convert fixed-form source files to free-form source files, by passing -ofree; it can also convert free-form to fixed-form by passing -ofixed, though this is (in my opinion) less useful. An extra command can be defined for each of these conversions if findent is selected as the formatter.

A design question: should conversion prompt the user to provide a new filename, with a default value that just changes the extension? For example, running a conversion on fixed-form file example.f, the extension would prompt the user for a new filename with example.f90 prefilled. Running the reverse conversion on example.f90 would prompt the user with the default of example.f. How should this behavior interact with the settings configured in "files.associations"?

Related: #956