Lint: Register ++/-- increment/decrement in UnsupportedRegistry (#300)

[PurHur]

Summary

• Map Expr_PreInc, Expr_PostInc, Expr_PreDec, Expr_PostDec to tracking issue Language: Pre/post increment and decrement (++/--) #137 in UnsupportedRegistry.
• Add IncrementDetector AST pass so phpc lint reports ++/-- even though php-cfg lowers them to assign+binary ops before compile.
• Extend LintTest, UnsupportedRegistryTest, and docs/unsupported-syntax.md.

Test plan

• vendor/bin/phpunit --filter 'UnsupportedRegistryTest|LintTest' (Docker php-compiler:22.04-dev)
• ./phpc lint -r '++$i;' prints #137
• php script/bootstrap-inventory.php --check

Closes #300

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