Fix start&top implementation in typechecker

[DavyLandman]

This PR continues the work in #2499 (that was merged to main a bit too soon) to fix #2496

• removes subtype relation between start[X] and X (in asubtype, alub, aglb)
• removes subtype relation between start symbols and regular symbols (lists)
• removes special cases for field selection and field update on .top
• removes special cases for \start non-terminal whereever the contextFreeSyntax role is expected by adding a role field to the start AType
• adds an implicit/automatic declaration, for every syntax rule for X that has a start modifier:
  • start production rule start[X] = X;
  • top field start[X] = X top;
  • layout wrapping start[X] = L X L;
• removes more special cases in the back-end
• removes missed special cases in the collector for pattern matching
• removed API functions isStart* and getStart* that were used for detecting the old special cases

Together these changes solve a small number of known issues with start symbols, and a larger number of previously undetected issues.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46%. Comparing base (53d2faa) to head (70daf1d).

Additional details and impacted files

@@           Coverage Diff           @@
##              main   #2551   +/-   ##
=======================================
- Coverage       46%     46%   -1%     
+ Complexity    6737    6727   -10     
=======================================
  Files          794     794           
  Lines        65923   65923           
  Branches      9888    9888           
=======================================
- Hits         30858   30836   -22     
- Misses       32684   32697   +13     
- Partials      2381    2390    +9     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[jurgenvinju]

Looks good. Needs an integration test.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud