Proposal: Annotation for DBML

# Annotation

## Current problems
* Every time new concepts are requested for DBML, we have to look into how those concepts work for other databases such as MySQL and PostgreSQL to define the syntaxes that can fit all.
* Some concepts cannot be supported in DBML because the concept mismatch across databases.

**Proposed solution:** These problems can be solved by annotation.

## Annotation
Annotations helps to associate metadata (information) to the DBML elements.

```
@ {
// syntax of each annotation is independent. For example, the syntax of
// dbdiagram annotation can be different from the syntax of dbdocs
// annotation
}
```

Example of dbdiagram annotation:
```
Table users {
id int [pk]
name varchar
}

@dbdiagram {
darkmode: on

Table users {
headercolor: #f3f2f9
position {
x: 181
y: 212
}
}
}
```

The example below fully demonstrate the power of annotation of supporting different database concepts without mixing concepts into general DBML code.
```
Table users {
id int [pk]
name varchar
status user_status

@postgresql { // annotation for this table users; use for exporting this table to PostgreSQL
id: [serial]
Indexes {
name: [type: gist]
}
}

@mysql { // annotation for this table users; use for exporting this table to MySQL
id: [auto_increment]
storage_type: memory
Indexes {
name: [type: hash]
}
}
}
```

Annotation can be placed anywhere in our DBML code. If it is placed inside an element, it will provide metadata for that element.

## Avantages of annotation
- **Annotation is a way to separate between high level data model (table, schema) and SQL low level implementation (indexes, storage type...)**
- One DBML file can contains many kinds of annotation without any problem. For example, one DBML table can be associated with two database annotations (mysql, postgres ... ) for exporting purpose without polluting the original DBML code
- Annotation syntax is independent. Anyone, any team can define their own annotation syntax and write the annotation parser plugin to parse them.
- From the view of DBML, annotation can be treated like comment. It does not affect the structure or the meaning of the database
- From the technical view, DBML dialect with syntax mixed into original DBML code is hard to implement and maintain. For annotation, we just need to write another specific annotation parser plugin.
- The render system in third-party applications such as dbdiagram and dbdocs can easily render diagrams/documents base on annotations.
- The possibilities is endless. Our DBML will be easier to extend without worry how the mix specific database syntax into the general code.