(Team 1) Design a Dictionary class for the DictionaryMatcher

[chenlica]

@sandeepreddy602 @inkudo @rajesh9625

We need this class to capture the concept of a dictionary. Please talk to me to do a discussion. Add @JavierJia as a reviewer.

[chenlica]

To increase the quality of the codebase, it will be good to add necessary high-level comments to the code (in addition to our documentation on wiki). For your next PR about adding spans to test cases of scan-based keyword search operator, please also take the chance to add high-level comments to some of the test cases and operators.

[sandeepreddy602]

These changes are part of the pull request #61