fix exposures crashing with referencing questions with multiple variables

[kkuznets]

Problem
Running dbt-metabase exposures produces faulty exposures.json if the referenced Metabase questions contain multiple variables in one query.

To reproduce
The problem is due to the greedy nature of the .* regex pattern that will catch multiple sets of double braces if they exist within the same line.

Example query:

SELECT *
FROM generate_series(  DATE_TRUNC ('day',{{start_date}}  - cast({{rolling_days}} || ' days' as interval)),  DATE_TRUNC ('day',now() - interval '1 day'), '1 day') as buckets(day);

Solution
Make the regex less greedy to match as little text as possible.

[gouline]

Nice! Can you please add a unit test that fails before your fix and succeeds after?

[kkuznets]

Sure! Added a simple unit test.