Fix Unicode.letter? to include all letter categories

scripts/generate_unicode_data.cr

@@ -220,7 +220,7 @@ alternate_ranges = alternate_ranges(downcase_one_ranges)
 casefold_ranges = case_ranges entries, &.casefold
 
 all_strides = {} of String => Array(Stride)
-categories = %w(Lu Ll Lt Mn Mc Me Nd Nl No Zs Zl Zp Cc Cf Cs Co Cn)
+categories = %w(Lu Ll Lt Lm Lo Mn Mc Me Nd Nl No Zs Zl Zp Cc Cf Cs Co Cn)
 
 categories.each do |category|
   all_strides[category] = strides entries, category, &.general_category

spec/std/char_spec.cr

@@ -82,6 +82,22 @@ describe "Char" do
     it { ' '.ascii_letter?.should be_false }
   end
 
+  it "#letter?" do
+    'A'.letter?.should be_true # Unicode General Category Lu
+    'a'.letter?.should be_true # Unicode General Category Ll
+    'Dž'.letter?.should be_true # Unicode General Category Lt
+    'ʰ'.letter?.should be_true # Unicode General Category Lm
+    'か'.letter?.should be_true # Unicode General Category Lo
+
+    'ः'.letter?.should be_false  # M
+    '1'.letter?.should be_false  # Nd
+    'Ⅰ'.letter?.should be_false  # Nl
+    '_'.letter?.should be_false  # P
+    '$'.letter?.should be_false  # S
+    ' '.letter?.should be_false  # Z
+    '\n'.letter?.should be_false # C
+  end
+
   describe "alphanumeric?" do
     it { 'a'.alphanumeric?.should be_true }
     it { 'A'.alphanumeric?.should be_true }

src/char.cr

@@ -239,6 +239,9 @@ struct Char
 
   # Returns `true` if this char is a letter.
   #
+  # All codepoints in the Unicode General Category `L` (Letter) are considered
+  # a letter.
+  #
   # ```
   # 'c'.letter? # => true
   # 'á'.letter? # => true