feat(core::props::Props): add function "get_ref"

[hasezoey]

ISSUE NUMBER - PULL_REQUEST_TITLE

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Description

This PR adds a get_ref function to Props to get the searched attribute by reference instead of always cloning like get does.
Also add tests for all set and get function of Props.

I did not include a get_ref_or function as it is simple enough to just chain props.get_ref(query).unwrap_or(default).

Personally, i would recommend to maybe remove the original get function in a major version and replace it with get_ref, this way it is up to the user when to clone the returned value.
If it should be removed, i would recommend to put a #[deprecated] on get before it being removed.

Type of change

Please select relevant options.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• My code follows the contribution guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I formatted the code with cargo fmt
• I checked my code using cargo clippy and reports no warnings
• I have added tests that prove my fix is effective or that my feature works
• I have introduced no new C-bindings
• The changes I've made are Windows, MacOS, UNIX, Linux compatible (or I've handled them using cfg target_os)
• I increased or maintained the code coverage for the project, compared to the previous commit

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This change would for example allow another clone at stdlib#TextSpan to be removed.
And likely other places too.

[hasezoey]

I also just noticed that MockComponent::query also returns a full AttrValue instead of a reference, maybe that could be changed too?