feat: @IsStrongPassword() converter

[invakid404]

This PR is a rough implementation of a converter for @IsStrongPassword(). It (ab)uses positive lookahead to represent the options of the constraint in terms of a regular expression.

All specified options are enforced using a similar regex pattern that looks like this:
(?=.*(?:[^...]*[...]){n})

This regular expression fragment is a positive lookahead that requires the presence of n occurrences of a certain pattern.

The pattern is defined as: [^...]*[...], where the [^...] represents any characters except for the characters within the brackets, and [...] represents the specific characters within the brackets. The * before the first brackets means zero or more occurrences of the first pattern.

Therefore, the entire pattern (?:[^...]*[...]) matches zero or more characters, followed by the specific characters within the brackets.

The lookahead (?=.*(?:[^...]*[...]){n}) requires that this pattern occur n times in the string, without actually consuming any characters in the string. The .* before the lookahead allows for any characters to occur before and between each occurrence of the pattern.

For example, if n=3 and [...] is replaced with a-z, the lookahead (?=.*(?:[^a-z]*[a-z]){3}) would require three occurrences of any lowercase character in the input, possibly separated by other characters.

This allows us to achieve exactly what we want: to check that certain characters are present at least a certain amount of times, without consuming the input. All of these so-called requirements are then wrapped in a basic expression that matches anything: ^.*$.

I've jumped the gun and decided that it makes sense for symbols to be configurable, as there's no universal definition for that.

Any suggestions that concern readability/maintainability are welcome :^)