slide syntax should support `[import:1]`

[azu]

Currently, support following patterns.

All: [import, hello-world.js](../src/hello-world.js)
1-2: [import:1-2, hello-world.js](../src/hello-world.js)
2-3: [import:2-3, hello-world.js](../src/hello-world.js)
2>=: [import:2-, hello-world.js](../src/hello-world.js)
<=3: [import:-3, hello-world.js](../src/hello-world.js)

We want to add following pattern:

1: [import:1, hello-world.js](../src/hello-world.js)

It is a alias of that

1: [import:1-1, hello-world.js](../src/hello-world.js)