Support the documentation of global variables

Following [this feature-request](OpenZeppelin#43), extending the `solidity-docgen` package to support the documentation of global variables, which are implicitly added by the compiler as getter functions in the contract.

This should allow users to get the natspec-documentation of their global variables added into the `solidity-docgen` output by extending the input template (hbs) file.

For example:
```
{{#if ownVariables}}
# Variables:
{{#ownVariables}}
- [`{{type}} {{name}}`](#{{anchor}})
{{/ownVariables}}
{{/if}}

{{#ownVariables}}
# Variable `{{type}} {{name}}` {#{{anchor~}} }
{{#if natspec.devdoc}}{{natspec.devdoc}}{{else}}No description{{/if}}
{{/ownVariables}}
```
Note that this PR is incomplete, since the `devdoc` of a global variable is always `undefined`.

For this reason, I also had to override function `SolidityContractItem.natspec()` in class `SolidityVariable` with a slightly different implementation (since `this.astNode.documentation === undefined`).

src/solidity.ts

@@ -99,6 +99,20 @@ export class SolidityContract implements Linkable {
     );
   }
 
+  get variables(): SolidityVariable[] {
+    return uniqBy(
+      flatten(this.inheritance.map(c => c.ownVariables)),
+      v => v.signature,
+    );
+  }
+
+  get ownVariables(): SolidityVariable[] {
+    return this.astNode.nodes
+      .filter(isVariableDeclaration)
+      .filter(n => n.visibility === 'public')
+      .map(n => new SolidityVariable(this, n));
+  }
+
   get functions(): SolidityFunction[] {
     return uniqBy(
       flatten(this.inheritance.map(c => c.ownFunctions)),
@@ -191,6 +205,30 @@ abstract class SolidityContractItem implements Linkable {
   }
 }
 
+class SolidityVariable extends SolidityContractItem {
+  constructor(
+    contract: SolidityContract,
+    protected readonly astNode: solc.ast.VariableDeclaration,
+  ) {
+    super(contract, astNode);
+  }
+
+  get type(): string {
+    return this.astNode.typeName.name;
+  }
+
+  get signature(): string {
+    return `${this.type} ${this.name}`;
+  }
+
+  get natspec(): NatSpec {
+    if (this.astNode.documentation) {
+      return parseNatSpec(this.astNode.documentation);
+    }
+    return {};
+  }
+}
+
 class SolidityFunction extends SolidityContractItem {
   constructor(
     contract: SolidityContract,
@@ -359,6 +397,10 @@ interface ToString {
   toString(): string;
 }
 
+function isVariableDeclaration(node: solc.ast.ContractItem): node is solc.ast.VariableDeclaration {
+  return node.nodeType === 'VariableDeclaration';
+}
+
 function isFunctionDefinition(node: solc.ast.ContractItem): node is solc.ast.FunctionDefinition {
   return node.nodeType === 'FunctionDefinition';
 }