request for new functions: `isvariable` and `variablename`

I want to be able to convert any TermInterface.jl expression to one of the expression types defined by my [CallableExpressions.jl](https://gitlab.com/nsajko/CallableExpressions.jl) package. However, I don't think that TermInterface.jl on its own currently makes that possible. The problem is that, for leaves of the expression tree, I'm not able to:

* tell variables apart from constants

* convert a *foreign* variable type to a *native* variable type, for that I need to associate the variable with a name, as each variable is determined by a single `Symbol` value in CallableExpressions.jl

I think it'd make sense for TermInterface.jl to introduce two additional functions:

* `isvariable` (name subject to bikeshedding, of course): `isvariable(x)` should be `true` when `x` is a single variable, and it should be `false` when `x` is a constant or an expression (neither constant expressions nor variable expressions count as "a variable" here)

* `variablename(x)` should return an object representing a unique name for the variable, ideally convertible to `Symbol`

Thoughts?