Arithmetic operator cannot be removed

[mbiesiad]

Describe the bug
Arithmetic operator cannot be removed using DEL button.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://engine.presearch.org/search?q=calculator
2. Type 2+ or 3 *
3. Try undo last typed char (in this case: + or *)
4. See issue

Expected behavior
Deleting arithmetic operator (+, -. *, /) is allowed, similar like currently for typed last number.

Additional context
It's a little bit similar like #124 but use case is little different.
Here user wants to remove the arithmetic mark to correct the last entered number or even change the operator (there, user accidentally entered an artitmetic wrongly and wants to easily change it).

Best wishes,

[Hkataya]

Hey @mbiesiad,
I implemented a fix for this issue and raised a PR for it. This is my first contribution to the project though so not sure who should review the PR.

[mbiesiad]

Hi Hkataya, sounds great! 🎉

[jejopl]

@Hkataya pushed your PR, good job 👍 It will be pushed to production in day or two