- We used to cache displayText, text document, position and completion triggers on the data field of a completion item however in practice this resulted in having a pretty massive footprint on how completion lists serialized/deserialized.
  - Because completion resolve used to cache more data and now we don't I added a telemetry event to capture how often a cache miss actually happens
  - As part of this I had to migrate the `CompletionListCache` to be synchronous so that we can synchronously resolve cache entries at completion resolve time.
- Updated tests to reflect new resolve behavior. As part of doing this I had to expand the completion resolve tests to actually generate completion lists so caches were properly updated. Ultimately this more accurately reflects what happens in real scenarios.
- Wanted to use records so added an `IsExternalInit.cs` file to enable record usage in netstandard2.0.

## Benchmark results

**Scenario:** Typing `@` in a Razor file and getting a basic C# completion list.

Optimized = true represents utilizing the Optimized completion list (which Roslyn does now) and unoptimized represents anyone who's consuming Roslyn's completion list who doesn't use the optimized completion list structure.

### Before (817KB)
|                                                    Method | Optimized |      Mean |     Error |    StdDev |   Op/s |     Gen 0 |    Gen 1 |    Gen 2 | Allocated |
|---------------------------------------------------------- |---------- |----------:|----------:|----------:|-------:|----------:|---------:|---------:|----------:|
| 'Completion List Roundtrip Serialization/Deserialization' |     False | 29.301 ms | 0.5816 ms | 0.6698 ms |  34.13 | 1718.7500 | 937.5000 | 250.0000 |    9.8 MB |
|                           'Completion List Serialization' |     False | 10.223 ms | 0.1926 ms | 0.1608 ms |  97.82 |  625.0000 | 250.0000 | 234.3750 |   3.27 MB |
|                         'Completion List Deserialization' |     False | 18.653 ms | 0.3708 ms | 0.5882 ms |  53.61 | 1062.5000 | 531.2500 |        - |   6.52 MB |
| 'Completion List Roundtrip Serialization/Deserialization' |      True | 19.509 ms | 0.3308 ms | 0.3094 ms |  51.26 | 1218.7500 | 718.7500 | 218.7500 |   7.03 MB |
|                           'Completion List Serialization' |      True |  2.121 ms | 0.0259 ms | 0.0216 ms | 471.46 |  199.2188 | 167.9688 | 164.0625 |    1.1 MB |
|                         'Completion List Deserialization' |      True | 18.869 ms | 0.3342 ms | 0.3849 ms |  53.00 | 1062.5000 | 531.2500 |        - |   6.52 MB |

### After (458KB)
|                                                    Method | Optimized |      Mean |     Error |    StdDev |   Op/s |     Gen 0 |    Gen 1 |    Gen 2 |  Allocated |
|---------------------------------------------------------- |---------- |----------:|----------:|----------:|-------:|----------:|---------:|---------:|-----------:|
| 'Completion List Roundtrip Serialization/Deserialization' |     False | 19.438 ms | 0.3835 ms | 0.3400 ms |  51.45 | 1093.7500 | 406.2500 | 125.0000 | 7216.06 KB |
|                           'Completion List Serialization' |     False |  8.355 ms | 0.0735 ms | 0.0651 ms | 119.68 |  437.5000 | 171.8750 | 125.0000 | 2839.71 KB |
|                         'Completion List Deserialization' |     False | 10.303 ms | 0.2045 ms | 0.5701 ms |  97.06 |  625.0000 | 312.5000 |        - | 3844.78 KB |
| 'Completion List Roundtrip Serialization/Deserialization' |      True | 10.847 ms | 0.2057 ms | 0.2021 ms |  92.20 |  656.2500 | 234.3750 | 109.3750 | 4458.84 KB |
|                           'Completion List Serialization' |      True |  1.163 ms | 0.0064 ms | 0.0060 ms | 860.16 |  123.0469 | 123.0469 | 123.0469 |  614.06 KB |
|                         'Completion List Deserialization' |      True | 10.103 ms | 0.2004 ms | 0.3909 ms |  98.98 |  625.0000 | 312.5000 |        - | 3844.78 KB |

### Results (Optimized = true)

**Roundtrip Improvement:** 1.8x
**Serialization Improvement:** 1.82x
**Deserialization Improvement:** 1.87x

Part of dotnet/aspnetcore#30232