Fix truncated environment variable expansions

[srchase]

Any environment variable expansions in smithy-build.json that did not start at the beginning of the field resulted in dropping the character that preceded the ${NAME} placeholder.

Consider the following smithy-build.json example, with the environment variable of FOO was set to BAZ:

{
    "version": "1.0",
    "projections": {
        "a": {
            "transforms": [
                {
                    "name": "includeShapesByTag",
                    "args": {
                        "tags": ["BAR${FOO}"]
                    }
                }
            ]
        }
    }
}

Currently, this results in the shapes with the tag BABAZ being included, instead of BARBAZ as expected.

This CR corrects the expansion behavior to assure preceding characters are included appropriately.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[mtdowling]

This works, but it would probably be easier to understand if this used https://github.com/awslabs/smithy/blob/master/smithy-utils/src/main/java/software/amazon/smithy/utils/SimpleParser.java. I think we can merge this for now to fix the bug, and then go back and improve this when we have time.