Extended checks for multiple assignments to the same feature

[JohannesMeierSE]

This PR is a follow-up of #1412 and #1766 and improves and extends the checks of multiple assignments (in the grammar) to the same feature/property (in the AST):

• Validate the mixed use of ?=, = and += for the same feature: A warning (no error) is shown, if values are assigned with different operators to the same feature. This is prone to data loss, among others.
• Contributed a new code action to replace the assignment operator = with +=.
• Fixed bugs for the existing validation, when there are multiple assignments with = (or ?=):
  • Create validation markers for critical assignments in other documents in the currently validated document instead (a called fragment might be located in another document).
  • Validate rewrite actions as starting point in order not to validate and mark them twice (rewrite actions might be located in fragments, which might be called twice).
  • Handle circular calls of fragments
• Wrote more test cases, in particular for the existing check for multiple assignments with ?= as assignment operator to the same feature.
• Both validations (1st for mixing ?=, =and +=, and 2nd for multiple assignments with ?= or =) got an issue code, which is used in the 2nd case for a corresponding, new code action (for = only, see above) and which will be relevant for serializing ASTs back to text in the future.