Document and configure running lldb from the command line (#5324)

The docs explain that you must use `--local-lldbinit` in the command
line, and include an example of how to run a file_test under lldb from
the command line.

This PR includes `.lldbinit` file and `lldbinit.py` file which set up
our default options, copied from the VSCode launcher.

The instructions include settin the `max-string-summary-length`, and we
include this in the vscode launcher for lldb, as printing `Dump()`
output can easily get truncated otherwise when printing an InstBlockId.

scripts/lldbinit.py

@@ -0,0 +1,28 @@
+#!/usr/bin/env python3
+
+"""Initialization for lldb."""
+
+__copyright__ = """
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+"""
+
+# This script is only meant to be used from LLDB.
+import lldb  # type: ignore
+import os
+
+project_root = os.path.dirname(os.path.realpath(__file__))
+
+ci = lldb.debugger.GetCommandInterpreter()
+result = lldb.SBCommandReturnObject()
+
+
+def RunCommand(cmd: str) -> None:
+    """Runs a command and prints it to the console to show that it ran."""
+    print("(lldb) %s" % cmd)
+    ci.HandleCommand(cmd, result)
+
+
+RunCommand(f"settings append target.source-map . {project_root}")
+RunCommand(f"settings append target.source-map /proc/self/cwd {project_root}")