Building many bicep files in an msbuild project seems slow

Reported by @marcre

We need to do some measurements to profile the execution of the CLI tool to see if the process launch overhead is dominating the compilation time.

Possible options assuming perf analysis justifies them:
1. Add bulk compilation arguments to CLI
2. Borrow the Roslyn approach of spinning up a compilation server for msbuild.
3. Take a dependency on Bicep.Core from the msbuild task assembly - Not recommended because it would force `net6` dependency on the msbuild task, which would break classic .net scenarios.

No files or tests are named. Start by profiling the CLI tool while building many Bicep files in an MSBuild project, then determine whether process-launch overhead dominates compilation time. Done means the measurements support a chosen approach among bulk compilation, a compilation server, or another justified option.

Written by the indexing model from the issue text.