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>

servobuild.example

@@ -14,6 +14,8 @@
 [build]
 # Set "mode = dev" or use `mach build --dev` to build the project with warning.
 # or Set "mode = release" or use `mach build --release` for optimized build.
+# Use `mode = <profile>` or `mach build --profile=<profile>` to build the given
+# profile. Check the `Cargo.toml` manifest for a complete list of custom profiles.
 # Defaults to prompting before building
 #mode = "dev"