Fix exception and warning for global variables

[Conduitry]

Fixes #2295. An exception and a warning about global variables were not raised when the variable was also referenced in the script. A couple of checks for presence in var_lookup have also been augmented to check whether the variable has global equal to true.

[codecov-io]

Codecov Report

Merging #2321 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2321   +/-   ##
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  Coverage   91.83%   91.83%           
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  Files           1        1           
  Lines          49       49           
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  Hits           45       45           
  Misses          4        4

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[Conduitry]

I'm actually not positive that the change in the second commit is something that we really want. If someone refers to a global foo in their <script>, do we really want to warn them about a {foo} reference in their template? They'd presumably already be getting warned about foo from their linter.

The first change I am sure we want to have, because we have no way to bind to a global like that.

[Rich-Harris]

I think we do want the warning, yeah — there are situations where there'll be no linter (e.g. in the REPL), and in any case linter warnings and compiler warnings happen in different phases so I don't think it'd be annoying