fix[startCase]: only uppercase the first letter of each word like lodash

src/compat/string/startCase.spec.ts

@@ -66,4 +66,15 @@ describe('startCase', () => {
     expect(startCase('top10th')).toBe('Top 10th');
     expect(startCase('1st place 2nd 3rd 4th')).toBe('1st Place 2nd 3rd 4th');
   });
+
+  it('should only uppercase the first letter of each word', () => {
+    expect(startCase('ПриветМир')).toBe('ПриветМир');
+    expect(startCase('αβγΔεζ')).toBe('ΑβγΔεζ');
+    expect(startCase('fooBAR')).toBe('Foo BAR');
+    expect(startCase('mIxEd CaSe')).toBe('M Ix Ed Ca Se');
+  });
+
+  it('should uppercase a first letter outside the Basic Multilingual Plane', () => {
+    expect(startCase('𐐨𐐩 deseret')).toBe('𐐀 𐐁 Deseret');
+  });
 });

src/compat/string/startCase.ts

@@ -27,11 +27,13 @@ export function startCase(str?: string): string {
       result += ' ';
     }
 
-    if (word === word.toUpperCase()) {
-      result += word;
-    } else {
-      result += word[0].toUpperCase() + word.slice(1).toLowerCase();
+    let firstLength = 1;
+
+    if (word.codePointAt(0)! > 0xffff) {
+      firstLength = 2;
     }
+
+    result += word.slice(0, firstLength).toUpperCase() + word.slice(firstLength);
   }
 
   return result;