Improve Code Quality of Tag Commands

[Eugenetanwl3881]

• Remove duplicated code that could been fetched from ParserUtil
• Added more assertions for defensive coding
• Add Javadocs missed out

[codecov-commenter]

Codecov Report

Merging #520 (9383929) into master (49e5a17) will decrease coverage by 0.10%. The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master     #520      +/-   ##
============================================
- Coverage     65.50%   65.40%   -0.11%     
+ Complexity      730      729       -1     
============================================
  Files           147      147              
  Lines          2554     2558       +4     
  Branches        288      288              
============================================
  Hits           1673     1673              
  Misses          743      743              
- Partials        138      142       +4     

Impacted Files	Coverage Δ
...arser/tagcommandparser/RenameTagCommandParser.java	83.33% <0.00%> (-10.79%)	:arrow_down:
src/main/java/seedu/foodrem/model/tag/Tag.java	63.63% <0.00%> (-14.15%)	:arrow_down:
src/main/java/seedu/foodrem/model/tag/TagName.java	86.66% <ø> (ø)
...em/model/tag/TagNameContainsKeywordsPredicate.java	0.00% <ø> (ø)
...em/model/tag/exceptions/DuplicateTagException.java	0.00% <ø> (ø)
...rem/logic/commands/tagcommands/TagCommandUtil.java	100.00% <100.00%> (ø)
...ogic/parser/tagcommandparser/TagCommandParser.java	90.00% <100.00%> (-0.91%)	:arrow_down:
...ic/parser/tagcommandparser/UntagCommandParser.java	90.00% <100.00%> (ø)

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