Refactor generating code to use full file templates with markers

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The code generation in version 3.1, uses multiple fprintf() statements to generate the output files. This makes the code hard to read and follow.

An approach having the full file template as a string would be easier to read. The template string could also have markers to trigger custom code generation. Markers could be formatted such as {pi} from the fmt library examples.

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Some status update about the issue:

• Markers are called Template Variables.
• Template variables format is ${name} where name is the name of the variable.
• The fmt library is not used in the project in favor of a custom implementation.
• The template variables expansion is handled by custom code. The code supports recursive variables. For example, let's say you have first-name=Luke, last-name=Skywalker and full-name=${first-name} ${last-name}. Processing the template string ${full-name} would expands to value Luke Skywalker.