Double quoted as string literal instead of identifier

[dai-chen]

Issue #, if available: #975

Description of changes: Change grammar, AST builder and documentation to make double quoted treated as a string literal rather than an identifier.

Documentation:

1. https://github.com/dai-chen/sql/blob/use-double-quoted-as-string/docs/user/general/datatypes.rst#string-data-types
2. https://github.com/dai-chen/sql/blob/use-double-quoted-as-string/docs/user/general/identifiers.rst#id1

Testing: UT and IT changed accordingly.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #974 (8a4376e) into develop (9a08770) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop     #974   +/-   ##
==========================================
  Coverage      99.87%   99.87%           
  Complexity      2381     2381           
==========================================
  Files            234      234           
  Lines           5477     5477           
  Branches         357      357           
==========================================
  Hits            5470     5470           
  Misses             5        5           
  Partials           2        2           

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[penghuo]

LGTM