Solved Issue #124 The underlines are not correctly displayed in RTL

[uniyeh]

Fixed the underline displaying issue by self-implementing reversing the text instead of reversing with TMPro.

Detailed implementation:

1. In RTLTextMeshPro.cs, removed all lines setting isRightToLeftText and added an enableRTL boolean member to replace it.
2. In RTLTextMeshProEditor.cs, rewrote DrawTextInput() to handle RTL text without setting values for m_IsRightToLeftProp and m_RtlText.
3. Deleted line 130 finalText.Reverse() in RTLTestMeshPro.cs since TMPro no longer reverses the text.
4. Rewrote the RichTextFixer.Fix(output) method to correct the order of RichTextTags by swapping the positions of <tag> and </tag>.

I also added a scene named "Underline" for reference.