feat(useFloor): new function (#1817)

packages/math/useFloor/index.md

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+---
+category: '@Math'
+---
+
+# useFloor
+
+Reactively Math.floor(value).
+
+## Usage
+
+```ts
+import { useFloor } from '@vueuse/math'
+
+const value = ref(45.95)
+const result = useFloor(value)
+```

packages/math/useFloor/index.test.ts

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+import { ref } from 'vue-demi'
+import { useFloor } from '.'
+
+describe('useFloor', () => {
+  it('should be defined', () => {
+    expect(useFloor).toBeDefined()
+  })
+  it('should work', () => {
+    const base = ref(45.95)
+    const result = useFloor(base)
+    expect(result.value).toBe(45)
+    base.value = -45.05
+    expect(result.value).toBe(-46)
+  })
+})

packages/math/useFloor/index.ts

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+import type { ComputedRef } from 'vue-demi'
+import { computed } from 'vue-demi'
+import type { MaybeComputedRef } from '@vueuse/shared'
+import { resolveUnref } from '@vueuse/shared'
+
+/**
+ * Reactively Math.floor(value).
+ *
+ * @see https://vueuse.org/useFloor
+ * @param base
+ * @param exponent
+ */
+export function useFloor(value: MaybeComputedRef<number>): ComputedRef<number> {
+  return computed<number>(() => Math.floor(resolveUnref(value)))
+}