getExportedDeclaration should work for all exports

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

Similar to what `getExportedDeclarations` returns, I'd like to easily grab any single export from a file to determine its references whether it's a variable, function, etc. I'm not sure if I'm doing something wrong, but right now `getExportedDeclaration` seems to only work for named exports (`export { identifier } from './file'`) and won't be called for any other exports.

**Describe the solution you'd like**

Using a simple theme file as an example, I'd like to easily grab the `theme` export:

```ts
// theme.ts
export const colors = {
primary: 'tomato',
grey: '#333',
}

export const baseTheme = {
icons: {
sm: '16px',
md: '24px',
lg: '32px',
},
} as const

export const theme = {
colors,
fontSizes: [32, 18, 14],
} as const

```

```ts
// script.ts
const project = new Project({
tsConfigFilePath: path.resolve(__dirname, 'tsconfig.json'),
})
const themeSourceFile = project.getSourceFile('theme.ts')
const themeDeclaration = themeSourceFile.getDeclarationOrThrow('theme')
const themeReferences = exportDeclaration.findReferences()
```

**Describe alternatives you've considered**

The way I'm doing this right now is through `getExportedDeclarations`:

```ts
let exportDeclaration: VariableDeclaration | undefined

for (const [name, declarations] of themeSourceFile.getExportedDeclarations()) {
if (name === 'theme') {
exportDeclaration = declarations[0] as VariableDeclaration
}
}
```

Start by reproducing the theme.ts and script.ts example, comparing getExportedDeclaration with getExportedDeclarations. Trace how getExportedDeclaration handles the named-export case and determine the expected result for the theme variable export. Done means a single export such as theme can be retrieved and used to find its references, with the existing behavior preserved.

Written by the indexing model from the issue text.