Refactored AST reflection

[spoenemann]

Closes #1184.

The new implementation of AstReflection no longer needs overriding any of the generic methods. All the necessary metadata is in the types field.

In addition, the new structure is useful for type-safe access to AST property names, e.g. reflection.types.MyType._myProperty. The underscore prefix is necessary to separate the property name constants from the metadata properties.

[spoenemann]

An alternative would be to move the property name constants to a separate structure so we can access them like reflection.props.MyType.myProperty. WDYT?

[ydaveluy]

Hey @spoenemann,

And what about prefixing the metadata properties name, properties and superTypes with $ to avoid the underscore prefix for the property name constants ?

Something like:

 BinaryExpression: {
            $name: BinaryExpression,
            $properties: {
                left: {
                    name: 'left'
                },
                operator: {
                    name: 'operator'
                },
                right: {
                    name: 'right'
                }
            },
            $superTypes: ['Expression'],
            // Property name constants
            left: 'left',
            operator: 'operator',
            right: 'right'
        },

[spoenemann]

Rethinking this, the property constants serve a very different purpose than the AstReflection service. The former are used in language-specific code to have stable access to AST types so you notice it with compiler errors when something has changed in the grammar spec. The latter is used for generic services that should work for all Langium languages.

I like @guiliannewiger's proposal in #1184 (comment) and updated this PR according to that.

I first thought it could be a problematic change to generate objects instead of string constants, but if you already have code like context.container.$type === Person, you'll get an error message:

This comparison appears to be unintentional because the types 'string' and '{ readonly $type: "Person" }' have no overlap.

The solution is to change the code to Person.$type. We should mark this very clearly in the CHANGELOG.

[msujew]

Looks great, thank you. I'll mention this in detail when writing the changelog for version 4.0 of Langium.

[msujew]

Suggestion: Do we want to add a create function in here as a factory method? This would resolve #1755. However, I'm fine with adding this later in a separate PR.

[spoenemann]

Wouldn't you just use normal object notation to create AST nodes programmatically?