bug(expr): to_char consider unsupported format as plain text

[KeXiangWang]

Describe the bug

Current, to_char works like this:

1. find all the format item like 'YYYY(means year)'
2. replace them with the number
3. if the format item is unrecognized, consider it as plain text and keep it unchaged.
4. return the string after replacemnt

The problem happens in 3. As we haven't support all the format item. An unknown format item will be kept as plain text and outputed. It does not produce any errors. This may cause user confused. Throwing an error may be better.

Error message/log

wrong output: `Day`

To Reproduce

SELECT to_char('2015-09-01T14:48:05.359000'::timestamp, 'Day');

Expected behavior

output Tuesday

How did you deploy RisingWave?

doesn't matter.

The version of RisingWave

v1.6

Additional context

No response

[fuyufjh]

Ping @KeXiangWang

[github-actions[bot]]

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