fix: iOS startMention check range (#278)

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# Summary

The `startMention` ref method on iOS takes wrong range into
consideration when checking for blocks and conflicts. Can't be
reproduced in the current state of main branch, but on the codeblocks
feature branch I was not able to add an indicator this way because a
codeblock was detected within default `NSRange` value `(0,0)`.

This PR puts a proper `textView.selectedRange` range in there.

## Test Plan

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## Screenshots / Videos

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## Compatibility

| OS      | Implemented |
| ------- | :---------: |
| iOS     |    ✅     |
| Android |    (doesn't apply)    |

Author: szydlovsky

ios/EnrichedTextInputView.mm

@@ -921,7 +921,7 @@ - (void)startMentionWithIndicator:(NSString *)indicator {
   MentionStyle *mentionStyleClass = (MentionStyle *)stylesDict[@([MentionStyle getStyleType])];
   if(mentionStyleClass == nullptr) { return; }
 
-  if([self handleStyleBlocksAndConflicts:[MentionStyle getStyleType] range:[[mentionStyleClass getActiveMentionRange] rangeValue]]) {
+  if([self handleStyleBlocksAndConflicts:[MentionStyle getStyleType] range:textView.selectedRange]) {
     [mentionStyleClass startMentionWithIndicator:indicator];
     [self anyTextMayHaveBeenModified];
   }