Could not load locales file when default export is not an ObjectExpression

Hi, just running into a small issue when using `svelte-intl-precompile`, when you have a locale file that declares a default export, it throws an error when the default export is a node of type "Identifier" (e.g. `const locale = {}; export default locale;` but works as expected when the node is the type "ObjectExpression" (e.g. `export default {}`).

Here's a minimal reproduction: https://stackblitz.com/edit/sveltejs-kit-template-default-jm1kse?file=locales/en.js

Here's the error
```
[svelte-intl-precompile] Could not load $locales/en (imported by $locales): /packages/app/src/locales/en.js: Cannot read propert
ies of undefined (reading 'forEach')
> Could not load $locales/en (imported by $locales): /packages/app/src/locales/en.js: Cannot read properties of undefined (readi
ng 'forEach')
at flattenObjectProperties (/node_modules/.pnpm/babel-plugin-precompile-intl@0.3.0/node_modules/babel-plugin-precompile-intl
/src/index.js:169:32)
at PluginPass.ExportDefaultDeclaration (/Users/connor/wk/ui/main/node_modules/.pnpm/babel-plugin-precompile-intl@0.3.0/node_modules/babel-plugin-pre
compile-intl/src/index.js:216:11)
at newFn (/node_modules/.pnpm/@babel+traverse@7.17.3/node_modules/@babel/traverse/lib/visitors.js:177:21)
at NodePath._call (/node_modules/.pnpm/@babel+traverse@7.17.3/node_modules/@babel/traverse/lib/path/context.js:53:20)
at NodePath.call (/node_modules/.pnpm/@babel+traverse@7.17.3/node_modules/@babel/traverse/lib/path/context.js:40:17)
at NodePath.visit (/node_modules/.pnpm/@babel+traverse@7.17.3/node_modules/@babel/traverse/lib/path/context.js:100:31)
at TraversalContext.visitQueue (/node_modules/.pnpm/@babel+traverse@7.17.3/node_modules/@babel/traverse/lib/context.js:103:1
6)
at TraversalContext.visitMultiple (/node_modules/.pnpm/@babel+traverse@7.17.3/node_modules/@babel/traverse/lib/context.js:72
:17)
at TraversalContext.visit (/node_modules/.pnpm/@babel+traverse@7.17.3/node_modules/@babel/traverse/lib/context.js:129:19)
at traverseNode (/node_modules/.pnpm/@babel+traverse@7.17.3/node_modules/@babel/traverse/lib/traverse-node.js:24:17)
```

This is the babel node that throws the error, notice `declaration: Node { type: 'Identifier' , ... }` is missing the "properties" property?
```
transforming (170) $locales{
node: Node {
type: 'ExportDefaultDeclaration',
start: 71,
end: 89,
loc: SourceLocation {
start: [Position],
end: [Position],
filename: undefined,
identifierName: undefined
},
declaration: Node {
type: 'Identifier',
start: 86,
end: 88,
loc: [SourceLocation],
name: 'en',
leadingComments: undefined,
innerComments: undefined,
trailingComments: undefined
},
leadingComments: undefined,
innerComments: undefined,
trailingComments: undefined
}
}
```

Works when node type is "ObjectExpression", because it has the "properties" property is an array.

```
transforming (95) $locales{
node: Node {
type: 'ExportDefaultDeclaration',
start: 124,
end: 197,
loc: SourceLocation {
start: [Position],
end: [Position],
filename: undefined,
identifierName: undefined
},
declaration: Node {
type: 'ObjectExpression',
start: 139,
end: 196,
loc: [SourceLocation],
properties: [Array],
extra: [Object],
leadingComments: undefined,
innerComments: undefined,
trailingComments: undefined
},
leadingComments: [ [Object], [Object], [Object], [Object], [Object], [Object] ],
innerComments: undefined,
trailingComments: undefined
}
}
```
Would be best to handle both types, but just noting it in documentation someplace (website or readme) would be good too.

Thank you :)

Start with babel-plugin-precompile-intl/src/index.js, especially flattenObjectProperties and the ExportDefaultDeclaration handler shown in the stack trace. Run the linked StackBlitz reproduction with a default export bound to an identifier, then verify that locale files using either an Identifier or ObjectExpression default export load successfully.

Written by the indexing model from the issue text.