Add MergeDocBlockStartFixer to normalize malformed /* docblocks

[TomasVotruba]

What

New MergeDocBlockStartFixer (docblock level) that fixes docblocks mistakenly opened with a single-asterisk /* instead of /**.

It:

• converts the /* opener to /**
• strips every empty *-only line in the block

Before / after

Leading empty line:

/*
 *
 * @return $this
 */

/**
 * @return $this
 */

Empty line between description and tags:

/*
 * Cleans query params saving url values.
 *
 * @param string $url
 */

/**
 * Cleans query params saving url values.
 * @param string $url
 */

Guards (no false positives)

• requires a phpdoc tag (@param/@return/…) → license/file headers left untouched
• requires at least one empty * line → clean /* blocks untouched
• inline single-line /* @var */ left to existing DoubleAsteriskInlineVarFixer

Tests

4 fixtures: 2 change cases + 2 skip guards (license header, no-empty-line block). PHPStan / Rector / ECS clean on changed files.