Change Lexer so that it supports different modes

[paupino]

For example, "index" is a valid identifier name within appropriate context however is also a first class keyword (INDEX) in other contexts.

[paupino]

An example of valid syntax is:

CREATE OR REPLACE FUNCTION index(index int) 
RETURNS int
AS $body$
SELECT index
$body$
LANGUAGE SQL;
SELECT index(5);

[paupino]

It's likely that I'll need to break out some tokens from others. For example what is not valid as a table name (for example):

• CREATE
• TABLE

What is valid:

• ALTER
• INDEX
• FUNCTION
• EXTENSION
• VIEW
• TYPE