svelte-check "unrecognized UnaryExpression operator 39" in reactive statement

[tyler-boyd]

Similar to #245

Describe the bug
When running svelte-check, I see the following warning:

Getting Svelte diagnostics...

Warning - unrecognized UnaryExpression operator 39!
                This is an edge case unaccounted for in svelte2tsx, please file an issue:
                https://github.com/sveltejs/language-tools/issues/new/choose
                 +$customFrom

To Reproduce

The code snippet is something like

$: if (+date !== +$customFrom) { ... }

The app seems to work fine, so I suspect this is a false-positive in the checker.

Expected behavior

No warnings, as this is valid code that compiles and runs correctly.

System (please complete the following information):

• OS: Linux, Mac
• IDE: N/A (but VSCode)
• Plugin/Package: svelte-check

Additional context
Again, seems similar to #245