Add `$ifExists` and `$cascade` to `dropTable()` methods

[vjik]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	✔️

See https://github.com/yiisoft/db/pull/880

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.49%. Comparing base (2cd93e1) to head (c6b2255). Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #319   +/-   ##
=========================================
  Coverage     98.48%   98.49%           
- Complexity      330      332    +2     
=========================================
  Files            21       21           
  Lines          1057     1061    +4     
=========================================
+ Hits           1041     1045    +4     
  Misses           16       16           

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