Fix indentation parsing (#830)

Inline snapshot testing attempts to figure out the indentation width of
a file by finding the first line with whitespace and grabbing the prefix
of whitespace. While this works fine if you trim whitespace from
whitespace-only lines, it breaks if you do not, because a line of the
form:

```
"    \n"
```

Will currently match in its entirety and incorporate the newline into
the indentation.

This PR fixes this by picking the first indented line that contains
non-whitespace characters, instead.

Sources/InlineSnapshotTesting/AssertInlineSnapshot.swift

@@ -388,8 +388,8 @@ public struct InlineSnapshotSyntaxDescriptor: Hashable {
       self.wasRecording = snapshots.first?.wasRecording ?? isRecording
       self.indent = String(
         sourceLocationConverter.sourceLines
-          .first(where: { $0.first?.isWhitespace == true && $0 != "\n" })?
-          .prefix(while: { $0.isWhitespace })
+          .first { $0.first?.isWhitespace == true && $0.contains { !$0.isWhitespace } }?
+          .prefix { $0.isWhitespace }
           ?? "    "
       )
       self.snapshots = snapshots