feat: ability to resolve refs using templating syntax

[pd93]

This PR adds the ability to resolve references to other variables using Golang's templating syntax.

For example:

version: '3'

tasks:
  ref-resolver:
    vars:
      MAP:
        map: {"name":"Alice","age":30,"children":[{"name":"Bob","age":5},{"name":"Charlie","age":3},{"name":"Diane","age":1}]}
      MAP_REF:
        ref: .MAP # <-- now requires "dot" syntax
    cmds:
      - task: print-var
        vars:
          VAR:
            ref: (index .MAP_REF.children 0).name # <-- use brackets and functions normally

  print-var:
    cmds:
      - echo "{{.VAR}}" # <-- echos "Bob"

Since this functionality is not exposed by the standard library, it depends on a fork of the text/template package and the changes in this PR.