Implement IFormattable on HslColor and HsvColor

[NathanDrake2406]

Is your feature request related to a problem? Please describe.

Follow-up to #18725 and #20919. Color is getting IFormattable support for configurable ToString formats. HslColor and HsvColor should get the same treatment for consistency.

Currently these types have a single ToString() that outputs a fixed format. There's no way to control alpha inclusion or output representation.

Describe the solution you'd like

Implement IFormattable on both HslColor and HsvColor with format specifiers following the same convention established in #20919: uppercase includes alpha, lowercase excludes it.

Proposed specifiers for HslColor:

Specifier	Output
H	hsl(h, s%, l%, a) — with alpha
h	hsl(h, s%, l%) — without alpha

Proposed specifiers for HsvColor:

Specifier	Output
V	hsv(h, s%, v%, a) — with alpha
v	hsv(h, s%, v%) — without alpha

Default ToString() behavior unchanged for both types.

Additional context

• Discussed in review of feat: implement unified IFormattable on Color, HslColor, and HsvColor #20919 by @robloo
• No shared files with feat: implement unified IFormattable on Color, HslColor, and HsvColor #20919, so can be developed in parallel
• Hex format specifiers (X/x/H/h) don't apply to HSL/HSV — hex is an RGB concept