restructure: Group related elements of each core module to maximize cohesion

[MrDave1999]

Requirements

• Classes that do not belong to any core module go in "Common".
• The directory structure of the test project must match the main project.
• The namespace will remain "YeSql.Net" in order not to introduce a breaking change.

Directory tree

.
└── src/
    ├── Loader/
    │   ├── YeSqlLoader.cs
    │   ├── YeSqlLoader.HelperMethods.cs
    │   ├── YeSqlLoaderException.cs
    │   └── SqlFile.cs
    ├── Parser/
    │   ├── YeSqlParser.cs
    │   ├── YeSqlParser.HelperMethods.cs
    │   ├── YeSqlParserException.cs
    │   └── Line.cs
    ├── Reader/
    │   ├── ISqlCollection.cs
    │   ├── YeSqlDictionary.cs
    │   ├── TagNotFoundException.cs
    │   └── ModelTag.cs
    └── Common/
        ├── Extensions/
        │   └── EnumerableExtensions.cs
        ├── Results/
        │   ├── YeSqlValidationResult.cs
        │   └── ResultOfT.cs
        ├── FormattingMessage.cs
        └── ExceptionMessages.cs