Getting a concrete definition: getDefinitionNodeOfKind(OrThrow)

**Is your feature request related to a problem? Please describe.**

I find myself almost always expecting to find a definition of a certain kind. I expect that most use-cases have such a need.

**Describe the solution you'd like**

From:

```ts
const elementDefinitions = element.getDefinitionNodes()
const classDeclaration = elementDefinitions.find(TypeGuards.isClassDeclaration)
if (classDeclaration == null) throw new Error(`All elements of @NgModule.imports are expected to reference a class.`)
```

To:

```ts
const classDeclaration = element.getDefinitionNodeByKindOrThrow(SyntaxKind.ClassDeclaration)
```

**Describe alternatives you've considered**

I can obviously wrap this myself in a function :smile:

Read CONTRIBUTING.md, then inspect the existing getDefinitionNodes() API and the SyntaxKind and TypeGuards usage shown in the issue. Determine the intended behavior for a matching definition and for no match, and verify that the new convenience API supports the class-declaration example without changing existing behavior.

Written by the indexing model from the issue text.