Improve reporting multiple issues for the exact same location

Some situations report multiple issues for the exact same location. They get reported as independant issues and it is hard to see that th e last three are probably just follow up errors of the first one:

error: Unexpected token: expected DataTypeDefinition but found KeywordAbstract
  ┌─ test.st:1:30
  │
1 │ CLASS TestClass METHOD foo : ABSTRACT END_METHOD END_CLASS
  │                              ^^^^^^^^ Unexpected token: expected DataTypeDefinition but found KeywordAbstract

error: Unexpected token: expected Datatype but found ABSTRACT
  ┌─ test.st:1:30
  │
1 │ CLASS TestClass METHOD foo : ABSTRACT END_METHOD END_CLASS
  │                              ^^^^^^^^ Unexpected token: expected Datatype but found ABSTRACT

error: Unexpected token: expected Literal but found ABSTRACT
  ┌─ test.st:1:30
  │
1 │ CLASS TestClass METHOD foo : ABSTRACT END_METHOD END_CLASS
  │                              ^^^^^^^^ Unexpected token: expected Literal but found ABSTRACT

error: Unexpected token: expected KeywordSemicolon but found 'ABSTRACT'
  ┌─ test.st:1:30
  │
1 │ CLASS TestClass METHOD foo : ABSTRACT END_METHOD END_CLASS
  │                              ^^^^^^^^ Unexpected token: expected KeywordSemicolon but found 'ABSTRACT'

We should try to group diagnostics by their location before we report them. Something like this:

error: Unexpected token: expected DataTypeDefinition but found KeywordAbstract (+ 3 additional)
  ┌─ test.st:1:30
  │
1 │ CLASS TestClass METHOD foo : ABSTRACT END_METHOD END_CLASS
  │                              ^^^^^^^^ 1. Unexpected token: expected DataTypeDefinition but found KeywordAbstract
  │                                       2. Unexpected token: expected Datatype but found ABSTRACT
  │                                       3. Unexpected token: expected Literal but found ABSTRACT
  │                                       4. Unexpected token: expected KeywordSemicolon but found ABSTRACT

Start by tracing how parser diagnostics are collected and rendered. Group diagnostics sharing an exact location, preserve the first as the primary message, and render the remaining diagnostics as additional messages; verify the output against the example in the issue.