Make isUpper (and variants) work for strings with non-alpha chars

The other variants are isLower, isUpperAscii and isLowerAscii

Fixes https://github.com/nim-lang/Nim/issues/7963.

This commit changes the behavior and signatures of:

- isUpper, isLower in the unicode module
- isUpperAscii, isLowerAscii in the strutils module

A second mandatory parameter skipNonAlpha is added to these 4 procs.

(This change affects only for the case where the input is a *string*.)

---

With skipNonAlpha set to true, the behavior mimics the Python isupper and
islower behavior i.e. non-alphabetic chars/runes are ignored when checking if
the string is upper-case or lower-case.

    Before this commit:

      doAssert(not isUpper("A B"))

    After this commit:

      doAssert(not isUpper("A B", false))    <-- old behavior
      doAssert isUpper("A B", true)

      Below two are equivalent:

                           isUpper("A B", true)

        isAlpha("A B") and isUpper("A B", false)

.. and the similar for other 3 procs.

changelog.md

@@ -42,6 +42,12 @@
 - ``math.`mod` `` for floats now behaves the same as ``mod`` for integers
   (previously it used floor division like Python). Use ``math.floorMod`` for the old behavior.
 
+- For string inputs, ``unicode.isUpper`` and ``unicode.isLower`` now require a
+  second mandatory parameter ``skipNonAlpha``.
+
+- For string inputs, ``strutils.isUpperAscii`` and ``strutils.isLowerAscii`` now
+  require a second mandatory parameter ``skipNonAlpha``.
+
 #### Breaking changes in the compiler
 
 - The undocumented ``#? braces`` parsing mode was removed.