feat: support nested vars

[aFlyBird0]

Signed-off-by: Bird aflybird0@gmail.com

Pre-Checklist

Note: please complete ALL items in the following checklist.

• I have read through the CONTRIBUTING.md documentation.
• My code has the necessary comments and documentation (if needed).
• I have added relevant tests

Description

Support:

vars:
  a: [[ b]]a
  b: 123
  c: [[a]]c
  d: [[a]]/[[ c ]]/[[b]]

It is equal to:

vars:
  a: "123a"
  b: 123
  c: "123ac"
  d: "123a/123ac/123"

Related Issues

New Behavior (screenshots if needed)

[daniel-hutao]

Good job!

Good unit tests!