`path` in bevy_reflect doesn't support traversing into a map

## What problem does this solve or what need does it fill?

`bevy_reflect` allows path strings, e.g.

```rust
let value = *foo.get_path::("d[0].value").unwrap();
```

The `d[0]` in the above path is indexing into the `0th` entry of the list-like field `d`.

There is no equivalent notation for indexing into a map-like field.

## What solution would you like?

```rust
// Maps use syntax {'key'} to differentiate from list syntax [idx].
let value = *foo.get_path::("m{'key'}.value").unwrap();
```

## What alternative(s) have you considered?

- It could use the same notation as lists, but that could be confusing, e.g.
```rust
let value = *foo.get_path::("m[key].value").unwrap();
```
- Maybe there's no need for the quotes?
```rust
let value = *foo.get_path::("m{key}.value").unwrap();
```
- I suppose not adding support for this at all is another option.

## Additional context

n/a

Start at the bevy_reflect get_path API and trace how the existing d[0] list traversal is parsed and resolved. Decide the map-path syntax from the alternatives in the issue, then add traversal for map-like fields and verify that a path such as m{'key'}.value resolves correctly.

Written by the indexing model from the issue text.