[Validator] Implement countUnit option for Length constraint

[spackmat]

Q	A
Branch?	6.3
Bug fix?	no
New feature?	yes
Deprecations?	no
Tickets	Fix #49309
License	MIT
Doc PR	symfony/symfony-docs#17948

This implements the countUnit option for the Length constraint as proposed and discussed in #49309. That allows to control which strlen-method is used to count the string length for the Length constraint:

• Length::COUNT_BYTES counts with \strlen()
• Length::COUNT_CODEPOINTS counts with \mb_strlen() (default setting reflecting the former behavior)
• Length::COUNT_GRAPHEMES counts with \grapheme_strlen() (e.g. emojis count as 1)

Also implements the corresponding tests.

Examples:

// count unicode characters like emojis by their bytes for your legacy database needs
new Length(max: 255, countUnit: Length::COUNT_BYTES);
// count unicode characters like emojis by their codepoints for your more modern database needs
new Length(max: 255); // that is the default like before or set explicitly to Length::COUNT_CODEPOINTS)
// count unicode graphemes (or ZWJ sequences) like emojis as 1 length for your fancy emoji-only microblogging service
new Length(max: 280, countUnit: Length::COUNT_GRAPHEMES);

[nicolas-grekas]

LGTM thanks, I just have minor comments.

[nicolas-grekas]

Please rebase+squash to get rid of merge commits.

[nicolas-grekas]

Thank you @spackmat.

[spackmat]

Thanks @nicolas-grekas and @stof! I'll provide a PR for the docs.