Better error message for invalid syntax followed by valid syntax

```julia-repl
julia> import a, b, c=d e, f, g, h
ERROR: ParseError:
# Error @ REPL[50]:1:17
import a, b, c=d e, f, g, h
# └─────────┘ ── extra tokens after end of expression

julia> 1 = 2 3
ERROR: ParseError:
# Error @ REPL[58]:1:6
1 = 2 3
# └┘ ── extra tokens after end of expression
```
I believe that the "extra tokens after end of expression" error should only occur if the expression preceding the extra tokens is complete and valid.

Maybe the real issue here is that this should throw?
```julia
julia> Meta.parse("import a = 1")
:(import a = 1)
```

Start by reproducing the two REPL examples and the Meta.parse("import a = 1") case described in the issue. Trace how JuliaSyntax classifies invalid syntax followed by additional tokens, then confirm the intended parse or error behavior with focused parser tests; done means the error message is emitted only for a valid preceding expression.

Written by the indexing model from the issue text.