RFR: 8373038: Interpolator factories should follow method naming convention

[Michael Strauß]

The following interpolator factories don't follow the standard method naming convention:

* `Interpolator.SPLINE(double, double, double, double)`
* `Interpolator.TANGENT(Duration, double, Duration, double)`
* `Interpolator.TANGENT(Duration, double)`
* `Interpolator.STEPS(int, StepPosition)`

New methods named `ofSpline`, `ofTangent`, and `ofSteps` are added. The existing methods are deprecated (not for removal) in favor of the correctly-named methods. This change is in line with the new `ofLinear` method added with #1977.

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Commit messages:
 - Interpolator factories should follow method naming convention

Changes: https://git.openjdk.org/jfx/pull/1996/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1996&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373038
  Stats: 135 lines in 7 files changed: 62 ins; 9 del; 64 mod
  Patch: https://git.openjdk.org/jfx/pull/1996.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1996/head:pull/1996

PR: https://git.openjdk.org/jfx/pull/1996