Allow grabbing theme values with a decimal (#815)

This PR contains a fix for spacing theme values containing a decimal,
like this:

```js
tw`pl-[calc(20px + theme(spacing.2.5))]`

// ↓ ↓ ↓ ↓ ↓ ↓

({
  "paddingLeft": "calc(20px + 0.625rem)"
});
```

Previously values like this would return an incorrect value.
This made square brackets required to select these type of values: 

```js
tw`pl-[calc(20px + theme(spacing[2.5]))]`
```

Related #814

src/core/lib/createTheme.ts

@@ -18,7 +18,16 @@ function createTheme(
     defaultValue?: string,
     options = {}
   ): number | boolean | Record<string, unknown> {
-    const [pathRoot, ...subPaths] = toPath(path)
+    let [pathRoot, ...subPaths] = toPath(path)
+
+    // Retain dots in spacing values, eg: `ml-[theme(spacing.0.5)]`
+    if (
+      pathRoot === 'spacing' &&
+      subPaths.length === 2 &&
+      subPaths.every(x => !Number.isNaN(Number(x)))
+    ) {
+      subPaths = [subPaths.join('.')]
+    }
 
     const value = getConfigValue(
       path ? ['theme', pathRoot, ...subPaths] : ['theme'],

tests/arbitraryValues.test.ts

@@ -229,6 +229,18 @@ it('should allow using the DEFAULT key when using theme', async () => {
   })
 })
 
+it('should allow dots instead of square brackets for decimal point values', async () => {
+  const input = 'tw`ml-[theme(spacing.0.5)]`'
+
+  return run(input).then(result => {
+    expect(result).toMatchFormattedJavaScript(`
+      ({
+        "marginLeft": "0.125rem"
+      });
+    `)
+  })
+})
+
 it('should not output unparsable arbitrary CSS values', async () => {
   // eslint-disable-next-line no-template-curly-in-string
   const input = 'tw`w-[${sizes.width}]`'