Improve DiscordBotTokenDetector and its tests.

[nuztalgia]

Fixes #627.

See the linked issue for more context. As mentioned, this is the "bare minimum" fix for the implementation. Lots of test cases have also been added and/or clarified.

[lirantal]

I've reached out to ChatGPT to explain the regex if it helps getting this PR merged quicker:

This is a regular expression used to match a specific pattern in a string. It is being passed to the re.compile() function, which compiles the regular expression into a pattern object that can be used for matching.

Here is a breakdown of the regular expression:

• r': This indicates that the string is a raw string, which means that backslashes are treated literally and do not need to be escaped.

• [MNO]: This matches any single character that is either M, N, or O.

• [a-zA-Z\d_-]: This matches any single character that is either a lowercase letter (a-z), an uppercase letter (A-Z), a digit (\d), or a hyphen or underscore (_).

• {23,25}: This indicates that the preceding character or character group (in this case, [a-zA-Z\d_-]) must be repeated 23 to 25 times.

• \.: This matches a literal period (.) character.

• [a-zA-Z\d_-]{6}\.: This matches any sequence of 6 characters that are either a letter, a digit, a hyphen, or an underscore, followed by a period.

• [a-zA-Z\d_-]{27}: This matches any sequence of 27 characters that are either a letter, a digit, a hyphen, or an underscore.

Overall, this regular expression is used to match strings that have the following form:

• The first character is either M, N, or O.
• The next 23 to 25 characters are either letters, digits, hyphens, or underscores.
• The next character is a period.
• The next 6 characters are either letters, digits, hyphens, or underscores.
• The next character is a period.
• The final 27 characters are either letters, digits, hyphens, or underscores.

For example, the string "Mabcdefghijklmnopqrstuvwxy.123456.abcdefghijklmnopqrstuvwxyz123" would match this regular expression.

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The regular expression itself has been changed only to support an additional O character (optional) and an optional longer string of up to 25 characters in length.

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This PR also includes more tests and is passing CI.
@syn-4ck it's a relatively small change to the regex to extend it to support other token formats. think we can land this?