Calculate shield_text_length in roads layer

[nvkelso]

1. While it's possible to calculate the text length of the shield_text property client side in both Tangram and MapboxGL, it's more performant to calculate that server side and include as a property on each relevant feature. Let's do that and call it shield_text_length as a post-processing step in queries.yaml with new transform function.
2. Because shield_text can be either numerical or string we should always store it as string values to also illuminate type conversions client side.
3. Shield artwork is between 1 and 6 chars long, so consider stripping out longer shields.

[nvkelso]

Closed via #1991 and #1994.