[CFE] Add benchmarking possibility to the incremental compiler

This CL allows the incremental compiler to pass a benchmarker and
perform benchmarking in the same way as is possible via `fasta compile`.
It's enabled by setting an environment variable, on linux this can be
done with

```
export DART_CFE_ENABLE_INCREMENTAL_COMPILER_BENCHMARKING=true
```

on Windows it would likely be something like

```
set DART_CFE_ENABLE_INCREMENTAL_COMPILER_BENCHMARKING=true
```

When set, benchmarking data is recorded and after a `computeDelta`
call the recorded data is printed.

Currently, visualization via the html document requires wrapping of
the output between

```
{"benchmarkers": [
```

and

```
]}
```

Change-Id: I78f1298ebc17c47a172abdf00c48dac388197d3b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/237693
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>