[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/ContainsOperator.box.txt

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