feat: support optional AS keyword in CTE

[reata]

Closes #198

[codecov]

Codecov Report

Merging #199 (fd78aa3) into master (dcc402e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #199   +/-   ##
=======================================
  Coverage   98.93%   98.93%           
=======================================
  Files          21       21           
  Lines         843      846    +3     
=======================================
+ Hits          834      837    +3     
  Misses          9        9           

Impacted Files	Coverage Δ
sqllineage/core/analyzer.py	100.00% <100.00%> (ø)
sqllineage/core/handlers/cte.py	100.00% <100.00%> (ø)
sqllineage/core/handlers/source.py	100.00% <100.00%> (ø)
sqllineage/utils/sqlparse.py	100.00% <100.00%> (ø)

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