Add Validation to Collection Variable Modal

[SoulKa]

Is your feature request related to a problem? Please describe.
The pattern detection for template variables in the request body (e.g. {{SOME_variable}}) currently uses \w+ as regex (only letters, digits and underscore). The collection variable modal however, allows setting any variable name as key (e.g. some-variable).

Describe the solution you'd like
We need validation for the variable keys to only allow the same regex used for the template variable detection. Either the "save" button is disabled for invalid input, or the input field only allows numbers, characters, and underscore.