Provide FEEL parsing capabilities

[pinussilvestrus]

Is your feature request related to a problem? Please describe.

#223 indicates we have problems with entering whitespaces in several places. Therefore this validation does not check for valid FEEL expressions.

Describe the solution you'd like

We should add a validation that indicates valid and invalid FEEL expressions. Therefore we would need to parse FEEL expressions and check for validity.

See the Zeebe Docs for getting more information about declaring FEEL expressions: https://docs.zeebe.io/reference/expressions.html#expressions-vs-static-values

Describe alternatives you've considered

Remove the space validation, which will be the first step of implementation. Every extensive FEEL validation will be handled inside this issue.

Additional Context

Related to #223
Related to #231