…e objects (nicolas-grekas)

This PR was merged into the 5.1-dev branch.

Discussion
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[String] add LazyString to provide memoizing stringable objects

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

Replaces #34190

The proposed `LazyString` class is a value object that can be used in type declarations of our libraries/apps.

Right now, when a method accepts or returns "strings|stringable-objects", the most accurate type declaration one can use is `string|object` (either in docblocks or in union types in PHP 8). The goal of `LazyString` is to allow one to use `string|LazyString` instead and gain type-accuracy, thus type-safety while doing so.

Another defining property of the proposed class is also that it memoizes the computed string value so that the computation happens only once.

Two factories are provided to create a `LazyString` instance:
- `LazyString::fromStringable($value): self` -> turns any object with `__toString()` into a `LazyString`
- `LazyString::fromCallable($callback, ...$arguments): self` -> delegates the computation of the string value to a callback (optionally calling it with arguments).

Two generic helpers are also provided to help deal with stringables:
- `LazyString::isStringable($value): bool` -> checks whether a value can be safely cast to string, considering `__toString()` too. This replaces the boilerplate we all have to write currently (`is_string($value) || is_scalar($value) || is_callable([$value, '__toString'])`)
- `LazyString::resolve($value): string` -> casts a stringable value into a string. This is similar to the casting `(string)` operator or to `strval()`, but it throws a `TypeError` instead of a PHP notice when a non stringable is passed. This helps e.g. with code that enabled strict types and want to maintain compatibility with stringable objects.

An additional feature of `LazyString` instances is that they allow exceptions thrown from the wrapped `__toString()` methods or callbacks to be propagated. This requires having the `ErrorHandler` class from the `Debug` or `ErrorHandler` components registered as a PHP error handler (already the case for any Symfony apps by default). As a reminder, throwing from `__toString()` is not possible natively before PHP 7.4.

Commits
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4bb19c6 [String] add LazyString to provide generic stringable objects