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GH-36720: [R] stringr modifier functions cannot be called with namespace prefix (#36758)

### Rationale for this change

Bug in implementation of string modified functions caused them to be swallowed when prefixed with stringr namespace

### What changes are included in this PR?

Strips out the `stringr::` prefix when expressions contain `stringr::fixed`, `stringr::coll`, `string::boundary` or `stringr::regex`

### Are these changes tested?

Yes

### Are there any user-facing changes?

Yes
* Closes: #36720

Lead-authored-by: Nic Crane <thisisnic@gmail.com>
Co-authored-by: Dewey Dunnington <dewey@dunnington.ca>
Signed-off-by: Nic Crane <thisisnic@gmail.com>