`SHOW` statements silently dropping quoted strings

When parsing generic `SHOW` statements such as `SHOW FUNCTIONS` or `SHOW PARAMETERS`, the parser creates a `ShowVariable` AST with each of the tokens after `SHOW` added as a `variable` node. This should allow functionality like `SHOW FUNCTIONS LIKE '%date%'` to be written by processing the AST even if the lexer doesn't understand what the query means.

However, this query `SHOW FUNCTIONS LIKE '%date%'` is parser to:

~~~json
[
{
"ShowVariable": {
"variable": [
{
"value": "FUNCTIONS",
"quote_style": null
},
{
"value": "LIKE",
"quote_style": null
}
]
}
}
]
~~~

Where the value in quotes is removed.

I think this is probably because `SHOW` queries, not followed by a well-known token (e.g. `SHOW VARIABLES`, `SHOW COLUMNS`) defaults to a variation of `SHOW VARIABLE`, and the tokens afterwards are expected to be formatted like Identifiers, but a quoted string doesn't match this expectation. If that is the case, I expect an error to be raised rather than silently dropping part of the query.

Thanks

SHOW FUNCTIONS LIKE '%date%' を再現し、ShowVariable AST を構築する汎用 SHOW ステートメントパーサーを追跡します。引用符で囲まれたトークンの処理を、示されている AST と比較します。引用符で囲まれた値が保持されるか、パーサーがその値を黙って破棄せずにエラーを報告すれば完了です。

索引モデルが issue の本文から書いたものです。