Wire up phpstan

[donatj]

• Wires up PHPStan.
• Sets the minimum version of PHP to 7.4
• Update phpunit to a safe version

[Copilot]

Pull request overview

This PR wires PHPStan into the project’s tooling/CI, raises the minimum supported PHP version to 7.4, and updates dev tooling dependencies to align with the new baseline.

Changes:

• Add PHPStan configuration and run it as part of make test.
• Bump minimum PHP requirement from 7.1 to 7.4 across docs/Composer/CI.
• Adjust PHPUnit dev dependency constraints to versions compatible with the new minimum PHP.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/DotNotationParser.php	Adds type hints for iterator keys and adjusts ParseException indexing logic.
README.md	Updates documented PHP minimum version to 7.4.
phpstan.neon	Introduces PHPStan config (max level, analyze src).
Makefile	Adds a phpstan target and runs it during make test.
composer.json	Raises PHP constraint to >=7.4, updates PHPUnit constraint, adds PHPStan dev dependency.
.github/workflows/ci.yml	Removes PHP 7.1–7.3 from the CI test matrix.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The ParseException message reports the unexpected character as occurring at $key (the closing quote position), but the exception index being passed is $nextKey (the position of the unexpected character after the quote). This mismatch makes debugging harder; use the same position (preferably $nextKey) consistently in both the message and the $charIndex argument.

Suggested change

	sprintf('failed to parse path, expected . or EOF, got "%s" at %d', $next, $key),
	sprintf('failed to parse path, expected . or EOF, got "%s" at %d', $next, $nextKey ?? $key ?? ($lastKey + 1)),