Accept VALUE as a synonym of VALUES only in MySQL

[LucaCappelletti94]

Every dialect accepted VALUE in place of VALUES, as in INSERT INTO t (a) VALUE (1). MySQL documents it ("VALUE is a synonym for VALUES in this context", https://dev.mysql.com/doc/refman/8.4/en/insert.html), but SQLite 3.51.1 and DuckDB 1.4.4 reject it with a syntax error, and the INSERT references of PostgreSQL, SQL Server, Snowflake, BigQuery, Redshift, Spark, ClickHouse and Oracle only list VALUES.

A new dialect method, supports_singular_value_keyword, defaults to false and is enabled for MySQL and the generic dialect. Every other dialect now rejects VALUE with Expected: SELECT, VALUES, or a subquery in the query body, found: VALUE. The existing parse_insert_values test runs its VALUE case on the dialects that keep it, and a new test checks both sides of the capability.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.25%. Comparing base (1aabd12) to head (f937872).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2662   +/-   ##
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  Coverage   81.25%   81.25%           
=======================================
  Files          42       42           
  Lines       33910    33920   +10     
  Branches    33910    33920   +10     
=======================================
+ Hits        27553    27563   +10     
  Misses       2792     2792           
  Partials     3565     3565           

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