Refactor Predicate classes into predicates folder

[Dominic-Khoo]

Refactored NameContainsKeywordsPredicate, TagContainsKeywordsPredicate and NricMatchesPredicate into predicates folder under person package.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ	Complexity Δ
...java/seedu/address/logic/commands/ViewCommand.java	84.21% <ø> (ø)	7.00 <0.00> (ø)
.../seedu/address/logic/parser/FindCommandParser.java	100.00% <ø> (ø)	5.00 <0.00> (ø)
...rson/predicates/NameContainsKeywordsPredicate.java	100.00% <ø> (ø)	7.00 <0.00> (?)
.../model/person/predicates/NricMatchesPredicate.java	100.00% <ø> (ø)	6.00 <0.00> (?)
...erson/predicates/TagContainsKeywordsPredicate.java	75.00% <ø> (ø)	5.00 <0.00> (?)

[reidenong]

LGTM!