Spark, Databricks: port upstream interval PRs #2614 and #2616 - #19

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This reverts commit ee51174.

Spark's multi-units interval syntax lets the string carry its own
units, as in INTERVAL '1 YEAR 2 DAYS 3 HOURS', so no unit follows the
literal. Databricks Runtime parses with the same grammar. A unit after
a literal is still read; the expression form this gives up
(INTERVAL 1 + 1 DAY) is not Spark syntax.

Address review: turning require_interval_qualifier off accepted
INTERVAL 1 and read SELECT interval FROM t as SELECT INTERVAL FROM AS t.
Keep it on and add supports_interval_string_without_qualifier: the value
must be a literal, and only an unsigned string may omit the qualifier.
INTERVAL is not reserved for these dialects, so SELECT interval FROM t
reads a column.

…ter interval is arithmetic

Address review: a double-quoted string is a Spark string literal, so it
may be an interval value and may omit the qualifier (INTERVAL "2 months",
INTERVAL "1" DAY). Only a bare number or a string after INTERVAL keeps
the interval error; a signed number falls back to the identifier, so
interval - 1 and INTERVAL -1 are arithmetic on the column, as in Spark.

A string window frame bound stays a string literal in these dialects
(RANGE BETWEEN '1 day' PRECEDING), as in Spark, rather than being read
as an interval without a qualifier.

Address review: Spark accepts INTERVAL '1' DAY TO as an alias, so assert
the error on TO x instead. parse_interval_number_without_unit_error and
parse_interval_expression_value repeat test_interval_numeric_requires_unit
and parse_interval_require_qualifier. The guard comment now covers
strings as well as numbers.

Spark's multi-units interval syntax lists several value / unit pairs,
INTERVAL 10 YEAR 20 MONTH, documented as the same as the string form
INTERVAL '10 YEAR 20 MONTH'. A new supports_interval_multi_units hook,
enabled for SparkSqlDialect and DatabricksDialect, reads the extra pairs
into that string form, so the AST is unchanged.

A double-quoted string is a Spark string literal, so it may be any value
of a multi-units interval, e.g. INTERVAL "1" DAY "2" HOUR.