Serialize tokens in expressions as get expressions

[1ec5]

A serialized expression should never contain token syntax. As of #11509 and #11452, if style JSON contains a text-field property set to a legacy function, the runtime styling getter for that property returns an expression containing strings with tokens in them. So simply getting a text-field then setting text-field to the same value will turn tokens into literal {token}s on screen.

For #11651, I’m working around this issue by implementing an “upgrade” step in Objective-C that replaces tokens with mgl_join: and key path expressions, but an mbgl implementation would be somewhat more performant and benefit more platforms.

/cc @anandthakker

[1ec5]

For #11651, I’m working around this issue by implementing an “upgrade” step in Objective-C that replaces tokens with mgl_join: and key path expressions

Unfortunately, #11651 didn’t address the reverse operation: setting a property to a constant string expression should’ve also prevented the legacy {token} syntax, perhaps by turning the string into a trivial concat expression. But #12402 is going to remove token string support from mbgl anyways.