Add LEN builtin function

[Darkclainer]

Fixes #474

I have few concerns about this PR:

• @asdine provided examples of using LEN with columns in tables. But this PR doesn't contain such tests, because tests will be required outside of expr package (or import of another parser will be needed). Anyway, I don't see anything bad in such test, but I don't known codebase well enough to decide is it worth it.
• Maybe error message can be improved.
• LEN function is quite expensive for ARRAY and DOCUMENT because it need to loop over elements. Probably this can be problem, but out of scope of the mentioned issue and this PR.

[codecov[bot]]

Codecov Report

Merging #482 (a35d713) into main (487d4b4) will decrease coverage by 0.03%. The diff coverage is 61.76%.

@@            Coverage Diff             @@
##             main     #482      +/-   ##
==========================================
- Coverage   79.15%   79.12%   -0.04%     
==========================================
  Files         128      128              
  Lines       10748    10782      +34     
==========================================
+ Hits         8508     8531      +23     
- Misses       1536     1545       +9     
- Partials      704      706       +2     

Impacted Files	Coverage Δ
internal/expr/functions/builtins.go	63.40% <61.76%> (-0.20%)	:arrow_down:
internal/stringutil/stringutil.go	73.52% <0.00%> (+5.88%)	:arrow_up:

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[Darkclainer]

Thank you for review, @asdine!

Actually, you can add tests in the sqltests directory. It is used to test complete sql queries and declare the expected output. Here is an example.

Here is what I suggest:

• add a file in sqltests/SELECT/len.sql
• move your tests in that file and adapt them to use a complete SELECT statement
• add the missing examples defined in the issue

Did it, now it seems more complete.

Maybe error message can be improved.

Do you have an example?

I do not. Actually, for me, any error in patched code seems like serious bug and should never be triggered, except for the first Eval that probably can fail. But may be you know situations when such cases can occur.