[R] bindings for str_count

[asfimport]

Reporter: Nicola Crane / @thisisnic
Assignee: Dragoș Moldovan-Grünfeld / @dragosmg

Related issues:

• [C++] Add substring count kernel (depends upon)
• [C++] Add regex count kernel (depends upon)

PRs and other links:

• GitHub Pull Request #11473

_{Note: This issue was originally created as ARROW-13156. Please see the migration documentation for further details.}

[asfimport]

Nicola Crane / @thisisnic:
Issue resolved by pull request 11473
#11473