fix(scan-operator): Improve typings using newer TS features

[cah-kyle-dunn]

Description:
The scan operator does not properly infer types when the output Observable type differs from the input Observable type (i.e. T != R). This issue was discussed at length a few years back in #2897. That PR was ultimately closed because there were unavoidable limitations due to the minimum supported TypeScript at that time. The current minimum TS version should allow these types to be improved.

Using issue #3150 as an example:

of('0').pipe(
  scan((acc, str) => acc + parseInt(str), 0)
);

The current types cause both acc and str to be inferred as a string type, but acc should be inferred to be a number from the seed value 0, which is what my change fixes.

Related issue (if exists):
#3150

[cah-kyle-dunn]

Closing in favor of #4598

[coveralls]

Pull Request Test Coverage Report for Build 8471

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• 5 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.2%) to 96.842%

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Files with Coverage Reduction	New Missed Lines	%
src/internal/scheduled/scheduleObservable.ts	1	92.31%
src/internal/scheduled/scheduled.ts	1	79.41%
src/internal/scheduled/scheduleIterable.ts	3	89.19%

Totals
Change from base Build 8469:	0.2%
Covered Lines:	5796
Relevant Lines:	5985

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💛 - Coveralls