Feature request: add posibility to pass regular expressions to the 'import-order' rules

Hi everyone,

Now we have the default groups, such as `["builtin", "external", "parent", "sibling", "index"]`, but would be better to add possibility to pass regular expressions to these groups definitions. For example, like this:

```
["buildin", "external", ".constants.js$", "index"]
```

It will improve code readability by adding customization of import groupings.
I just want to get approve from maintainers of this repository. Its not a problem to implement it and create PR by myself.

Start by locating the import-order rule implementation and its existing tests. Read how the current default groups are parsed and matched, then determine the expected behavior for regular-expression group definitions. Done means custom expressions such as ".constants.js$" consistently control import grouping and the relevant tests pass.

Written by the indexing model from the issue text.