[PowerAssert] Correctly align infix calls for built-in operators

Instead of searching for the operator in the string representation of
the whole expression, consider the operator's start to be the
first non-whitespace non-dot character _after_ the LHS of the infix
expression.

This fixes cases like this:
```
assert("Name in " in listOf("Hello", "World"))
             |       |
             |       [Hello, World]
             false
```

^KT-66208 Fixed

plugins/power-assert/testData/codegen/operator/NegativeContainsOperator.box.txt

@@ -6,17 +6,17 @@ assert("Hello" !in listOf("Hello", "World"))
 
 Assertion failed
 assert(" !in " !in listOf(" !in "))
-         |         |
-         |         [ !in ]
-         false
+               |   |
+               |   [ !in ]
+               false
 
 Assertion failed
 assert(
     " !in "
-      |
-      false
 
             !in
+            |
+            false
 
             listOf(" !in ")
             |
@@ -25,14 +25,14 @@ assert(
 
 Assertion failed
 assert("Hello"/*!in*/!in/*!in*/listOf("Hello", "World"))
-       |                       |
-       |                       [Hello, World]
-       false
+               |               |
+               |               [Hello, World]
+               false
 
 Assertion failed
 assert(("Hello" !in listOf("Hello", "World")) !in listOf(false))
-                |   |                             |
-                |   |                             [false]
+                |   |                         |   |
+                |   |                         |   [false]
+                |   |                         false
                 |   [Hello, World]
-                false
                 false