feat: downlevel .d.ts for older TS versions

[L2jLiga]

allows to use spectator 5.x with Angular 8 with TS 3.5

Signed-off-by: Andrey Chalkin L2jLiga@gmail.com

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngneat/spectator/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[x] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Angular 8 with TS 3.5 tests fails due to TS errors like

node_modules/@ngneat/spectator/lib/spectator/spectator.d.ts:15:9 - error TS1086: An accessor cannot be declared in an ambient context.

15    get component(): C;

Issue Number: #285

What is the new behavior?

.d.ts are downleveled to build without any errors

get component(): C; -> readonly component: C; (accessors were added in TS 3.6)

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[NetanelBasal]

It should also close #285, right?

And did you test that this works?

[L2jLiga]

Right, I tested on my quite huge Angular 8 project with TS 3.5

[NetanelBasal]

Thanks