Provide a distinction between reserved and unreserved identifiers

[usadson]

Describe the bug
At the moment, reserved and unreserved identifiers are categorized under the same type: Keyword. This is disadvantageous because of the extend to which this enumeration is built. For example, commonly used identifiers such as id, number, etc. are recognized as keywords, when in practice they're used as colomn names and table names.

To Reproduce
Steps to reproduce the behavior:

CREATE TABLE table1 (
  id INT,
  number INT
);

Expected behavior
These identifiers should only be disallowed in certain contexts, but not in these common cases.

[usadson]

Fixed in commit d0ca37c