feat(editor): Quick fixes, completion and navigation while typing

Quick fixes
- Errors get a red quick-fix bulb; Apply Quick Fix (⌥⇧↵) fixes the most
  severe problem at the caret, and the error hover offers the fix directly.
- Code generated by a code action is re-indented to the file's style.
- Python: Implement abstract methods (⌃I or the bulb) from the bases.
- Duplicate code actions are listed once.

Completion
- Overloads sharing a name, and a name importable from several modules,
  are no longer merged into one entry.
- Auto-import entries show the module they import from.
- Call snippets without a stop (`new ArrayList<>()`) leave the caret inside
  the parentheses with parameter info, or past them when nothing takes
  arguments; parameter info also opens after server call snippets.
- The selected item is resolved ahead of acceptance, so its auto-import
  is applied (rust-analyzer).

Navigation and diagnostics
- Language server requests wait for pending edits, so Go to Declaration
  right after typing answers for the current text; an empty answer right
  after an edit is retried while the server catches up.
- Diagnostics come back after a window reload; error underlines stay
  visible under typo / unused highlighting.
- Implementation gutter markers no longer refresh in a loop while the
  server reports progress.
- A missing alternative server no longer shows as a server error.

src/editor/lsp/pythonMembers.test.ts

@@ -0,0 +1,50 @@
+import { describe, expect, it } from "vitest";
+import { classMembers, enclosingClass, memberStubs } from "./pythonMembers";
+
+const src = `from abc import ABC, abstractmethod
+
+
+class Shape(ABC):
+    @property
+    @abstractmethod
+    def name(self) -> str: ...
+
+    @abstractmethod
+    def area(
+        self, scale: float = 1.0
+    ) -> float:
+        """Area."""
+
+    def describe(self) -> str:
+        return self.name
+
+
+class Square(Shape):
+    def __init__(self, side: float):
+        self.side = side
+`.split("\n");
+
+describe("python members", () => {
+  it("finds the class enclosing a line", () => {
+    expect(enclosingClass(src, 4)).toBe(4);
+    expect(enclosingClass(src, 13)).toBe(4);
+    expect(enclosingClass(src, 21)).toBe(19);
+    expect(enclosingClass(src, 1)).toBeNull();
+  });
+
+  it("lists methods with their decorators and signatures", () => {
+    const members = classMembers(src, 4);
+    expect(members.map((m) => [m.name, m.abstract])).toEqual([
+      ["name", true],
+      ["area", true],
+      ["describe", false],
+    ]);
+    expect(members[0]!.lines).toEqual(["@property", "def name(self) -> str:"]);
+    expect(members[1]!.lines).toEqual(["def area(", "    self, scale: float = 1.0", ") -> float:"]);
+  });
+
+  it("writes stubs at the body's indentation", () => {
+    const [name] = classMembers(src, 4);
+    expect(memberStubs([name!], "    ", "    ")).toBe("    @property\n    def name(self) -> str:\n        pass");
+  });
+});