[Setup] Non Apple platforms (#13)

This PR contains the work done to address the issue #12, with regards to provide basic support of non Apple platforms.

To provide further details about the work done:
- [x] flattened the folder structure, especially now that the idea to filter folders based on platform is being discarded;
- [x] implemented precompiler processors to filter out platform-specific source code;
- [x] updated the `Package` file to provide basic support for non-Apple platforms;
- [x] added and also improved some targets to the `Makefile` file to smooth the current development workflows;
- [x] updated the `.gitignore` file with references to the `.vscode` folder and the `.env` file;
- [x] updated the Swift tools version to v5.7.

Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Reviewed-on: #13

Makefile

@@ -0,0 +1,87 @@
+# --- IMPORTS ---
+
+# Imports environment variables.
+environment_vars ?= .env
+
+include $(environment_vars)
+export $(shell sed 's/=.*//' $(environment_vars))
+
+# --- ARGUMENTS ---
+
+override docker?=${CLI_USE_DOCKER}
+override tag?=${DOCKER_IMAGE_TAG}
+override platform?=${DOCKER_IMAGE_PLATFORM}
+override config?=${SWIFT_BUILD_CONFIGURATION}
+override clean?=${DOCKER_IMAGE_CLEAN}
+
+# --- DEPENDENCIES ---
+
+outdated: ## List the package dependencies that can be updated.
+	@swift package update --dry-run
+
+update: ## Update the package dependencies defined in the project.
+	@swift package update
+
+# --- DEVELOPMENT ---
+
+build: ## Build this package with Swift either locally or in a Docker image.
+ifeq ($(docker),yes)
+	@-docker run \
+		--rm \
+		--volume ${PWD}:${DOCKER_VOLUME_TARGET} \
+		--workdir ${DOCKER_VOLUME_TARGET} \
+		--platform ${platform} \
+		${DOCKER_IMAGE_NAME}:${tag} \
+		swift build --configuration ${config}
+    ifeq ($(clean),yes)
+		@docker rmi ${DOCKER_IMAGE_NAME}:${tag}
+    endif
+else
+	@swift build --configuration ${config}
+endif
+
+# --- TESTING ---
+
+test: ## Test this package with Swift either locally or in a Docker image.
+ifeq ($(docker),yes)
+	@-docker run \
+		--rm \
+		--volume ${PWD}:${DOCKER_VOLUME_TARGET} \
+		--workdir ${DOCKER_VOLUME_TARGET} \
+		--platform ${platform} \
+		${DOCKER_IMAGE_NAME}:${tag} \
+		swift test --configuration ${config}
+    ifeq ($(clean),yes)
+		@docker rmi ${DOCKER_IMAGE_NAME}:${tag}
+    endif
+else
+	@swift test --configuration ${config}
+endif
+
+# --- HOUSE-KEEPING ---
+
+clean: ## Clean the build artifacts of the package.
+	@swift package clean
+
+reset: ## Reset the build folder of the package.
+	@swift package reset
+
+flush-images: ## Flush all outstanding Swift docker images.
+	@docker images \
+		--all | grep ${DOCKER_IMAGE_NAME} | awk '{print $$3}' | xargs docker rmi --force
+
+# --- MISCELLANEOUS ---
+
+xcode: ## Open this package in Xcode.
+	@open -a Xcode Package.swift
+
+# --- HELP ---
+
+# Outputs the documentation for each of the defined tasks when `help` is called.
+# Reference: https://marmelab.com/blog/2016/02/29/auto-documented-makefile.html
+.PHONY: help
+
+help: ## Prints the written documentation for all the defined tasks.
+	@awk 'BEGIN {FS = ":.*?## "} /^[a-zA-Z_-]+:.*?## / {printf "\033[36m%-30s\033[0m %s\n", $$1, $$2}' $(MAKEFILE_LIST)
+
+.DEFAULT_GOAL := help