Discard non-variables within double quoted strings

[stronk7]

This change fixes a couple of things:

• Stop detecting some literals (escaped $) as variable names.
• Process multiple variables in the same string. Previously only the first variable found was being checked.

Fixes #10

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (42ab278) 96.61% compared to head (807358d) 96.61%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #105 +/- ## ========================================= Coverage 96.61% 96.61% - Complexity 537 538 +1 ========================================= Files 23 23 Lines 1537 1538 +1 ========================================= + Hits 1485 1486 +1 Misses 52 52 ```

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