Update text decorations. Fix 'mention' generator and emoji offsets.

[JrooTJunior]

Description

• Updated text decorations
• Fixed mention generation with nested entities @username
• Fixed emoji offset

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• unit tests
• manually

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Merging #315 into dev-3.x will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           dev-3.x      #315   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          210       210           
  Lines         3982      4014   +32     
=========================================
+ Hits          3982      4014   +32     

Flag	Coverage Δ
#unittests	100.00% <100.00%> (ø)

Impacted Files	Coverage Δ
aiogram/utils/markdown.py	100.00% <100.00%> (ø)
aiogram/utils/text_decorations.py	100.00% <100.00%> (ø)