feat: add support for Coq

[tchajed]

Coq is an interactive theorem prover developed by INRIA. It's widely used for formalized mathematics, proving metaproperties of programming languages, and program proofs.

Coq is supported by linguist and has about 5k GitHub stars.

This is the syntax sample I wrote for shiki, as highlighted by GitHub:

From Coq Require Import ssreflect.

(* ensure proofs are well-structured *)
Set Default Goal Selector "!".
#[global] Open Scope general_if_scope.

Module list_playground.
  (* Let's do a typical proof by induction: we'll define [list] as an inductive,
     [app] (list append) as a recursive function, and prove that [app] is
     associative. *)
  Inductive list (A: Type) :=
  | nil
  | cons (x: A) (l: list A).

  (* Fix up implicit arguments. *)
  Arguments nil {A}.
  Arguments cons {A} x l.
  Notation "[]" := nil.
  Infix "::" := cons.

  Fixpoint app {A} (l1 l2: list A): list A :=
    match l1 with
    | [] => l2
    | x :: l1 => x :: app l1 l2
    end.

  Infix "++" := app.

  Theorem app_assoc {A} (l1 l2 l3: list A) :
    (l1 ++ l2) ++ l3 = l1 ++ (l2 ++ l3).
  Proof.
    induction l1 as [|x l1]; simpl.
    - reflexivity.
    - by rewrite IHl1.
  Qed.
End list_playground.

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