Add npos const static member to cudf::string_view

[davidwendt]

Adds npos member to cudf::string_view for consistency with std::string_view::npos https://en.cppreference.com/w/cpp/string/basic_string_view/npos
This is used mainly in the find() and rfind() functions which should return npos when the target string is not found. Code updates are included in this PR to use npos instead. This change will help when building and documenting strings-udf functions that use find() and rfind().
Also moved UNKNOWN_STRING_LENGTH into the string_view class scope since it is only used internally by string_view.
No actual code behavior has changed.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (branch-22.08@36ec9a7). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff               @@
##             branch-22.08   #11166   +/-   ##
===============================================
  Coverage                ?   86.30%           
===============================================
  Files                   ?      144           
  Lines                   ?    22698           
  Branches                ?        0           
===============================================
  Hits                    ?    19589           
  Misses                  ?     3109           
  Partials                ?        0           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 36ec9a7...266a318. Read the comment docs.

[bdice]

Approving with small questions/comments.

[davidwendt]

@gpucibot merge