[fea-rs] Support inline contextual positionals

Here's an example from the AFDKO spec:

```
markClass sukun @TOP_CLASS;

lookup MARK_POS {
position base lam_meem_jeem' mark @TOP_CLASS alef;
} MARK_POS;
```

This causes:

```
error: Expected ';'
in foo.fea at 4:31
|
4 | position base lam_meem_jeem' mark @TOP_CLASS alef;
| ^

error: ''' Not valid in a lookup block
in foo.fea at 4:31
|
4 | position base lam_meem_jeem' mark @TOP_CLASS alef;
| ^

error: Expected = found ID
in foo.fea at 4:67
|
4 | position base lam_meem_jeem' mark @TOP_CLASS alef;
| ^^^^

error: Expected named glyph class or '['.
in foo.fea at 4:71
|
4 | position base lam_meem_jeem' mark @TOP_CLASS alef;
```

The issue identifies fea-rs but names no file or test. Start by reproducing the AFDKO example and tracing the parser errors for the inline contextual positional; done means the example parses without the reported errors.

Written by the indexing model from the issue text.