This commit fixes/adds tests for and fixes several issues with `JOIN`
operator parsing:
- For OUTER joins, LEFT | RIGHT | FULL specifier is not optional
```nim
doAssertRaises(SqlParseError): discard parseSql("""
SELECT id FROM a
OUTER JOIN b
ON a.id = b.id
""")
```
- For NATURAL JOIN and CROSS JOIN, ON and USING clauses are forbidden
```nim
doAssertRaises(SqlParseError): discard parseSql("""
SELECT id FROM a
CROSS JOIN b
ON a.id = b.id
""")
```
- JOIN should parse as part of FROM, not after WHERE
```nim
doAssertRaises(SqlParseError): discard parseSql("""
SELECT id FROM a
WHERE a.id IS NOT NULL
INNER JOIN b
ON a.id = b.id
""")
```
- LEFT JOIN should parse
```nim
doAssert $parseSql("""
SELECT id FROM a
LEFT JOIN b
ON a.id = b.id
""") == "select id from a left join b on a.id = b.id;"
```
- NATURAL JOIN should parse
```nim
doAssert $parseSql("""
SELECT id FROM a
NATURAL JOIN b
""") == "select id from a natural join b;"
```
- USING should parse
```nim
doAssert $parseSql("""
SELECT id FROM a
JOIN b
USING (id)
""") == "select id from a join b using (id );"
```
- Multiple JOINs should parse
```nim
doAssert $parseSql("""
SELECT id FROM a
JOIN b
ON a.id = b.id
LEFT JOIN c
USING (id)
""") == "select id from a join b on a.id = b.id left join c using (id );"
```
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