Add `isGradientFunction(name: string): boolean` to `css-analyzer/values`

[bartveneman]

Context

css-analyzer/values already exports classification helpers like colorFunctions, colorKeywords, and namedColors. There is no equivalent helper for identifying CSS gradient functions.
Consumers currently write inline regex checks like:

/^(repeating-)?(linear|conic|radial)-gradient$/.test(name)

This duplicates knowledge of which function names are gradients and will need updating if new gradient types are standardized (e.g. conic-gradient was added relatively recently).
Proposed API

/** Returns true if `name` is a CSS gradient function name. */
function isGradientFunction(name: string): boolean

Example

import { isGradientFunction } from '@projectwallace/css-analyzer/values'

isGradientFunction('linear-gradient')          // true
isGradientFunction('repeating-radial-gradient') // true
isGradientFunction('conic-gradient')           // true
isGradientFunction('rgb')                      // false
isGradientFunction('url')                      // false

Gradient functions to cover

• linear-gradient
• radial-gradient
• conic-gradient
• repeating-linear-gradient
• repeating-radial-gradient
• repeating-conic-gradient

Motivation

Centralises the list of gradient function names alongside other value-classification constants in this package. Prevents consumers from maintaining inline regexes that may diverge from the actual CSS specification over time.