Support for async codegen and Sourcery 2.2.3

[art-divin]

Related to krzysztofzablocki/Sourcery#1308

Context

This PR adds facilities to support concurrent codegen based on swifttemplate

Details

In order to be able to generate the code concurrently for every type, shared state needs to be removed.

This PR does the following:

1. Removes all the shared state
2. Update Sourcery version to 2.2.3
3. Small improvements / refactoring

[markst]

This could probably overrule #352

[markst]

seems to be failing for me:

[art-divin]

Hey @markst , actually the screenshot looks fine. There's a print statement below, and the contents of the file on the screenshot is not the final code which would go into the generated mock

[markst]

We might also need to update kSourceryVersion in Global.swift for the CLI:
https://github.com/MakeAWishFoundation/SwiftyMocky/blob/master/Sources/CLI/Core/Utils/Global.swift#L5

[markst]

I've built from this branch multiple times:

swift build --disable-sandbox -c release

but using the build binary:

./swiftymocky generate --disableCache --verbose

Insists on the template including characters.split which fails post Swift 5:

❌  Error: ShellOut encountered an error
Status code: 3
Message: "Building for production...
[0/5] Write sources
[2/5] Write swift-version-33747A42983211AE.txt
[4/7] Emitting module SourceryRuntime
[5/7] Compiling SourceryRuntime AccessLevel.swift
[6/9] Emitting module SwiftTemplate
[7/9] Compiling SwiftTemplate main.swift
/private/var/folders/qv/k4c8t3_95tzc96m0cpv30tn80000gq/T/SwiftTemplate/EF30FB5F-CF6A-4725-B7E7-65C3340E92C4/2.2.2/Sources/SwiftTemplate/main.swift:251:96: error: 'characters' is unavailable: Please use String directly
            split($0, byFirstOccurenceOf: " where ").0.replacingOccurrences(of: " ", with: "").characters.split(separator: ":").map(String.init).first
                                                                                               ^~~~~~~~~~
Swift.String:5:16: note: 'characters' was obsoleted in Swift 5.0

I can't work out where this template is coming from, it's determined not to use the template in your branch despite building from checked out branch.

[markst]

Ah! it's because we need to update the base64 encoded files at Assets.swifttemplate.mock which are used for the CLI

[markst]

think we're just missing an opening tag:

Suggested change

	func generate(type: Type, methodRegistrar: MethodRegistrar, subscriptRegistrar: SubscriptRegistrar) async -> String {
	<%_
	func generate(type: Type, methodRegistrar: MethodRegistrar, subscriptRegistrar: SubscriptRegistrar) async -> String {