It's a great idea, but I don't think defining the syntax of a programming language as a syntax.toml file will work for enough programming languages for this to be useful. You're basically rewriting the parser of your language in a DSL that isn't as expressive as the language the parser is written in.

I think you'd need another parser/syntax interface for this to work. E.g. running a binary that you can submit source code to which responds with a JSON file containing the parsed tokens. That way you can reuse the compiler's parser.