Add further sanitation when receiving config data in code

Problem/Motivation

Currently, in the "Search404Controller" config values are not sanitized enough. This is especially the case for "search404_ignore_extensions" and "search404_ignore_query".

Steps to reproduce

Proposed resolution

Taken from @Anybody inside https://git.drupalcode.org/project/search404/-/merge_requests/12:

We might be so kind here to

1. Trim the string first - always a good thing in such a case
2. Remove dots (str_replace)
3. Also split by "," which isn't untypical
4. Trim the exploded results and check them to be not empty (if someone adds two spaced for example)

Alternatively, we could NOT do that and instead add a validation which checks for:

1. dots
2. two spaces
3. or simply any non a-z characters?

Same validation helper should be used for search404_ignore_query.

Remaining tasks

User interface changes

API changes

Data model changes