Add `SiblingToken`s back to the parser

[psteinroe]

Sibling tokens are tokens that should end up at the same depth in the CST, such as ( and ). Right now, that is not the case. Take for example the following statement:

ALTER TABLE products ALTER COLUMN price TYPE numeric(10,2);

The brackets of numeric are not on the same level:

[email protected]
        [email protected] "ALTER"
        [email protected] " "
        [email protected] "TABLE"
        [email protected] " "
        [email protected]
          [email protected] "products"
        [email protected] " "
        [email protected]
          [email protected] "ALTER"
          [email protected] " "
          [email protected] "COLUMN"
          [email protected] " "
          [email protected] "price"
          [email protected] " "
          [email protected] "TYPE"
          [email protected] " "
          [email protected]
            [email protected]
              [email protected] "numeric"
              [email protected] "("  <-- this is deeper than
              [email protected]
                [email protected] "10"
              [email protected] ","
              [email protected]
                [email protected] "2"
        [email protected] ")" <-- this
        [email protected] ";"

To fix this, we have to keep track the depth at which tokens are "opened". If the closing token is encountered, we apply it at the same depth as its sibling.

A SiblingToken struct already exists from past implementations. It should be reused.