We should have a code fix for BCP045 (can't use "+" with strings)

```bicep
var s1 = 'hello'
var s2 = 'there'
output o string = s1 + s2
```
```
Cannot apply operator "+" to operands of type "'hello'" and "'there'". Use string interpolation instead.bicep(BCP045)
```

Start with the BCP045 diagnostic shown in the issue and reproduce the example using the two string variables and the + operator. Trace where this diagnostic is produced and where code fixes are registered; done means the example offers a fix that replaces concatenation with string interpolation.

Written by the indexing model from the issue text.