fix: allowing skyline keywords as columns, tables, etc.

[nicky6s]

Following on from this PR -> #2078

Associated to this issue -> #2136

The keywords LOW, HIGH, PLUS and INVERSE were missing from line 2048 of JSqlParserCC.jjt as they were added as new restricted keywords. I have added them manually to line 2048 and removed them as restricted keywords.

I could have done the same with PREFERRING, but since #2078 was merged its become an important new keyword like WHERE, so Ive left it out.

[manticore-projects]

Greetings! Thank you for your contribution and good job overall.

However, the grammar has some ambiguities now:

         line 3461, column 11 and line 3464, column 11 respectively.
         A common prefix is: "HIGH" "NOT"
         Consider using a lookahead of 3 or more for earlier expansion.
Warning: Choice conflict involving two expansions at
         line 3462, column 11 and line 3464, column 11 respectively.
         A common prefix is: "LOW" "NOT"
         Consider using a lookahead of 3 or more for earlier expansion.
Warning: Choice conflict involving two expansions at
         line 3463, column 11 and line 3464, column 11 respectively.
         A common prefix is: "INVERSE" "("
         Consider using a lookahead of 3 or more for earlier expansion.

Please fix by inserting the advised LOOKAHEADS. Then I would merge.

[nicky6s]

@manticore-projects Thanks for reviewing

Please fix by inserting the advised LOOKAHEADS

Ive made this change -> 9eeb92f

[manticore-projects]

Good job, thank you for your contribution!