add support for s-expression trees

[monperrus]

Hi, it would be great to have support for s-expressions.

We might implement our own parser, or reuse some code:

• https://github.com/antlr/grammars-v4/blob/master/sexpression/sexpression.g4
• @julianmendez's https://github.com/julianmendez/jsexp

cc/ @jacarte

[jrfaller]

Also this one : https://github.com/AbstractMachinesLab/tree-sitter-sexp