Using Substance variables not declared in Style header

[Yiliang Liang]

Describe the bug

For references to back-ticked variables `x` in Style block body, the compiler doesn't check whether or not x really exists as a Substace variable, nor does it check that `x` is declared in the Style block header.
To Reproduce

https://penrose.cs.cmu.edu/try/?gist=55abb34c7d501de0c03ac70d94bc71ed

Expected behavior

There should be a compiler error since not_in_header has not been declared in the Style block header. Furthermore, not_in_header is not even a Substance variable.

[Yiliang Liang]

IMO the behavior should be: the body of a Style block can only access Substance variables through the Style header.

So, `abc`.xyz should only be allowed in Style block body if `abc` (as a Substance variable) is somehow declared in the Style header.