Add medium profile for daily work scenario (#34035)

- The standard `dev` mode is often too slow for daily work on servo
- Increasing the optimization level to 2 improves the performance noticably.
- Build time for opt-level = 1 is slightly higher than 2 on my machine
- Reducing debug info to `line-tables-only` improves link and build times,
  while still keeping relevant info for backtraces.

Also extend .servobuild to allow setting custom cargo profiles instead
of just release and dev.

Signed-off-by: Jonathan Schwender <jonathan.schwender@huawei.com>

python/servo/command_base.py

@@ -678,6 +678,10 @@ class CommandBase(object):
             elif self.config["build"]["mode"] == "release":
                 print("No build type specified, but .servobuild specified `--release`.")
                 return BuildType.release()
+            elif self.config["build"]["mode"] != "":
+                profile = self.config["build"]["mode"]
+                print(f"No build type specified, but .servobuild specified custom profile `{profile}`.")
+                return BuildType.custom(profile)
             else:
                 print("No build type specified so assuming `--dev`.")
                 return BuildType.dev()