Allow alias name beginning with AS

[oshadmi]

Allow alias name to begin with AS instead of failing

127.0.0.1:6379> FT.CREATE idx SCHEMA a TEXT 

127.0.0.1:6379> FT.SEARCH idx * RETURN 3 a AS asa
(error) Alias for RETURN cannot be `AS`

127.0.0.1:6379> FT.AGGREGATE idx * LOAD 3 a AS asa
(error) Alias for RETURN cannot be `AS`

[codecov]

Codecov Report

Base: 82.98% // Head: 82.98% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (89132be) compared to base (3f95dcb).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3259      +/-   ##
==========================================
- Coverage   82.98%   82.98%   -0.01%     
==========================================
  Files         181      181              
  Lines       30171    30171              
==========================================
- Hits        25038    25037       -1     
- Misses       5133     5134       +1     

Impacted Files	Coverage Δ
src/aggregate/aggregate_request.c	96.42% <100.00%> (ø)
src/spec.c	87.92% <0.00%> (-0.07%)	⬇️

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