MySQL allows integer interval values

The usual format for an interval is `INTERVAL 'value' unit`, but MySQL allows `'value'` to be an expression that is either a string or a number. For example:
```
SELECT DATE_SUB('2018-05-01',INTERVAL '1' YEAR);
SELECT DATE_SUB('2018-05-01',INTERVAL 1 YEAR);
```
both return `2017-05-01`. Currently, the latter syntax is not supported.

Start by locating the parser entry point and existing tests for SQL INTERVAL expressions. Compare the supported quoted-value form with MySQL's numeric-value form, then add coverage showing that both examples parse successfully.

Written by the indexing model from the issue text.