Config versioning

[chicoxyzzy]

We can add versioning to the config, so when people update the version of ssc and run it, we can know the version of config.ini is not compatible with the version of ssc and throw a readable error and maybe even provide an update script.

I think we can add an ungrouped field like this:

[config]
revision: 20220130

[build]
# ...

[meta]
# ...

Then,

• if ssc wants a newer version of config, we print something like Current version of config is outdated and not compatible with your version of 'ssc'
• if ssc version is too old, we print Current version of config is newer than installed 'ssc' can use. Please update your version of 'ssc'.

Automatic updates

We can add something like ssc config update to automatically update the config version

"Migrations" should provide the change for every field in such manner

// from non-versioned to 20220130

// [] build -> [build] script; identity
// [] env -> [build] env; identity
// [] executable -> x
// [] flags -> [build] flags; identity
// [] headless -> [build] headless; identity
// [] name -> [build] name; identity
// [] input -> [build] input; identity
// [] output -> [build] output; identity
// [] bundle_identifier -> [meta] bundle_identifier; identity
// [] copyright -> [meta] copyright; identity
// [] description -> [meta] description; identity
// [] file_limit -> [meta] file_limit; identity
// [] lang -> [meta] lang; identity
// [] maintainer -> [meta] maintainer; identity
// [] title -> [meta] title; identity
// [] version -> [meta] version; identity

Left value is a current value which we should move or delete
Right value is a new field and optional mapping function when we need to change a value (the mapping function isstd::identity by default)

This way we can create a chain of transformations for each config field for any old version to the latest version

Comments above reflect the recent changes made in #124