Convenient method to avoid adding named imports multiple times

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

Thanks for ts-morph!!!

I'm generating code, and I'd like a quick way to avoid adding a named import multiple times.

It's possible to check for imports via `getDeclaration()`:

```
// given a name (named import) and a path
if (!sourceFile.getImportDeclaration(path)) {
sourceFile.addImportDeclaration({moduleSpecifier: path, namedImports: [name]})
```

I'd like to go a step further and make sure the named import isn't already added by doing

```
if (!sourceFile.getImportDeclaration(path).getNamedImports(name))
sourceFile.addImportDeclaration({moduleSpecifier: path, namedImports: [name]})
```

I couldn't see how to make a dummy `ImportSpecifier` to search with `Array.includes()` on the result of `getNamedImports()` but I assume it's an internal class which makes sense.

**Describe the solution you'd like**

Maybe a way to ask of a `"name"` or `{name: "name", alias: "alias"}` is already declared in the sourceFile?

**Describe alternatives you've considered**

The following works to prevent adding multiple times, but perhaps I'm missing something for it to be simpler (always learning with JS and TS):
```
const dcl = sourceFile.getImportDeclaration(path)
if (!dcl || dcl.getNamedImports().filter((n) => n.getName() === name).length === 0) {
sourceFile.addImportDeclaration({ moduleSpecifier: path, namedImports: [name] })
}
```

Read CONTRIBUTING.md, then inspect the existing SourceFile APIs getImportDeclaration(), getNamedImports(), and addImportDeclaration() described in the issue. Define a convenient way to check whether a named import is already present, including the requested name or alias form, and verify that repeated additions are prevented.

Written by the indexing model from the issue text.