Langium grammar can declare AST elements with conflicting names

[RGHenderson]

Langium version: 0.3.0
Package name: langium

Steps To Reproduce

1. Create a minimal grammar containing an entry rule, and reference to a rule of name 'Error'
2. Run langium generator3.
3. Run build
4. See error something like:

src/language-server/generated/ast.ts:59:27 - error TS2351: This expression is not constructable.
  Type 'String' has no construct signatures.

59                 throw new Error(`${referenceId} is not a valid reference id.`);

Which makes sense given the content of the generated ast.ts:

export interface Error extends AstNode 
...
export const Error = 'Error';

Minimal grammar which causes error:

grammar ErrorExample

entry ErrorExample:
    errors+=Error;

Error:
    'error' name=ID;

terminal ID: /[_a-zA-Z][\w_]*/;

The current behavior

Langium allows the definition of conflicting AST element names, causing the above build error.

The expected behavior

Either the generated AST will be more smart, OR langium grammars are validated against reserved rule names.

[msujew]

Thanks for the report @RGHenderson! We should probably add reserved names for built-in JavaScript types. The only other option would be to rename them automatically, which would be quite unintuitive for users.