fix: do not try to find unresolved variables of a broken expression

[barmac]

Closes #50

Proposed Changes

Currently, we force-try to parse a broken expression. This PR takes advantage of strict parsing to bail out in case a syntax error is encountered.

The issue cannot be reproduced in the npm start environment, but you can link it to Desktop Modeler. Below is the demo of a diagram opened without errors:

Screen.Recording.2025-09-26.at.14.54.48.mov

Checklist

To ensure you provided everything we need to look at your PR:

• Brief textual description of the changes present
• Visual demo attached
• Steps to try out present, i.e. using the @bpmn-io/sr tool
• Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}

[barmac]

@marstamm Since you implemented most parts of this library, I'd appreciate it if you could have a look at the change.

[marstamm]

Nice, simple fix for large unparsable expressions. No comments from my side 👍

[barmac]

Released as 1.3.5