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

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.

Start at the css-analyzer/values entry point and inspect the existing classification helpers and exports. Add the proposed isGradientFunction API for the six listed gradient names, then verify those names return true while rgb and url return false.