Update E999 to show all syntax errors (#11900)

## Summary

This PR updates the linter to show all the parse errors as diagnostics
instead of just the first one.

Note that this doesn't affect the parse error displayed as error log
message. This will be removed in a follow-up PR.

### Breaking?

I don't think this is a breaking change even though this might give more
diagnostics. The main reason is that this shouldn't affect any users
because it'll only give additional diagnostics in the case of multiple
syntax errors.

## Test Plan

Add an integration test case which would raise more than one parse
error.

crates/ruff_python_parser/src/lib.rs

@@ -274,11 +274,11 @@ impl<T> Parsed<T> {
 
     /// Returns the [`Parsed`] output as a [`Result`], returning [`Ok`] if it has no syntax errors,
     /// or [`Err`] containing the first [`ParseError`] encountered.
-    pub fn as_result(&self) -> Result<&Parsed<T>, &ParseError> {
-        if let [error, ..] = self.errors() {
-            Err(error)
-        } else {
+    pub fn as_result(&self) -> Result<&Parsed<T>, &[ParseError]> {
+        if self.is_valid() {
             Ok(self)
+        } else {
+            Err(&self.errors)
         }
     }
 

crates/ruff_python_parser/tests/fixtures.rs

@@ -126,8 +126,7 @@ fn test_invalid_syntax(input_path: &Path) {
 #[allow(clippy::print_stdout)]
 fn parser_quick_test() {
     let source = "\
-def foo()
-    pass
+from foo import
 ";
 
     let parsed = parse_unchecked(source, Mode::Module);