fix(transloco): 🐛 Add generic type default to Translate<T>()

[seadonk]

PR Checklist

Please check if your PR fulfills the following requirements:

• [X ] The commit message follows our guidelines: https://github.com/ngneat/transloco/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?

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

What is the current behavior?

Translate function is generically typed as unknown by default.
This causes type errors including:

Argument of type 'unknown' is not assignable to parameter of type 'string'

Issue Number: #544

What is the new behavior?

Translate function is generically typed as string by default.

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

[seadonk]

@shaharkazaz Sorry, accidentally closed the PR, then reopened it.