Add @param string literal union when only literals are passed

[TomasVotruba]

When an untyped parameter only ever receives plain string literals, add string plus a @param docblock with the exact values:

+/**
+ * @param 'eq'|'gt'|'neq' $operator
+ */
-public function compareScore(int $score, $operator)
+public function compareScore(int $score, string $operator)

Rules:

• 2 to 10 distinct literals, single-quoted, sorted
• null also passed -> ?string + 'a'|'b'|null
• any non-literal string (constant, sprintf(), interpolation, escapes) skips the docblock
• a string default must be one of the literals, otherwise skipped
• existing @param for the parameter is kept; a redundant @param string is replaced
• only for params argtyper types itself, already typed params are left alone