Fixing letterOf and positionOf to use Arrays as lengthOf does

[jguille2]

Hi!

It is related to #3183

If finally we keep current "Arrays operation" with "length of string" block, this PR fixes "letter of" and "position of" to be coherent with it.

• "Letter of" is simple, because it's only adding that "Array.from(string)" pre-transformation.
• "Position of" (from "strings" library) has more changes... We don't need any "JS primitive", but we have to consider our "blocks cases" to build a coherent operation (if a field is empty...)

There are more blocks affected, from "strings" and "word-sentence" libraries... but all based on "letterOf" and "positionOf" blocks

Because here (github PR) custom block changes are not easily shown, I paste that block here:

[cycomachead]

Thanks! This LGTM. We should have done this a while ago. :)