Add Aliasing support to ResiReg

[mkeoliya]

What this does

This PR adds aliasing to ResiReg, namely 3 commands:

• alias : Adds an alias. Format: alias c/<command> a/<alias> e.g. alias c/rooms a/r
• dealias : Deletes an alias. E.g. dealias c/rooms a/r
• aliases : Lists all aliases. Format: aliases

How to test

1. Launch the application.
2. Type in the above 3 commands.
3. Note that you can't have the same alias for different commands, an alias cannot be a command word, and undo/redo don't apply to alias commands (because they do not change the state of ResiReg).

Notes

Added a lot of testing in this PR. Note to team-mates: please add Integration tests for the commands you've added to ResiReg by this weekend - it takes a LOT of work.

Fixes #103

[codecov-io]

Codecov Report

Merging #101 into master will increase coverage by 4.72%.
The diff coverage is 85.44%.

@@             Coverage Diff              @@
##             master     #101      +/-   ##
============================================
+ Coverage     55.74%   60.46%   +4.72%     
- Complexity      539      655     +116     
============================================
  Files           109      121      +12     
  Lines          2185     2446     +261     
  Branches        259      289      +30     
============================================
+ Hits           1218     1479     +261     
+ Misses          904      901       -3     
- Partials         63       66       +3     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/seedu/resireg/Main.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...main/java/seedu/resireg/commons/core/Messages.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
src/main/java/seedu/resireg/model/Model.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...resireg/logic/parser/DeleteAliasCommandParser.java	8.33% <8.33%> (ø)	1.00 <1.00> (?)
...c/main/java/seedu/resireg/logic/CommandMapper.java	82.05% <36.36%> (-17.95%)	4.00 <1.00> (ø)
...edu/resireg/storage/JsonSerializableUserPrefs.java	72.72% <72.72%> (ø)	2.00 <2.00> (?)
...rc/main/java/seedu/resireg/model/ModelManager.java	80.39% <75.00%> (+11.42%)	40.00 <7.00> (+12.00)
...va/seedu/resireg/storage/JsonUserPrefsStorage.java	80.00% <78.57%> (-7.50%)	7.00 <5.00> (+3.00)	⬇️
src/main/java/seedu/resireg/model/alias/Alias.java	80.00% <80.00%> (ø)	8.00 <8.00> (?)
...in/java/seedu/resireg/model/alias/CommandWord.java	83.33% <83.33%> (ø)	9.00 <9.00> (?)
... and 46 more

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[chloelee767]

LGTM!