feat: parse templates in collection-type variables

[pd93]

Fixes #1477
Fixes #1511

This PR overhauls the way templating works in Task. Instead of having methods on templater.Template, we now have package-level functions instead. This allows us to use generics (as generic methods aren't possible right now) which vastly simplifies the calling API. This is especially useful for the "Any Variables" experiment since many types of variable may need to support templating.

For example, we now have the ability to support collection-type variables (slices/maps) and these might contain strings in which we want to use templates. See example below:

version: '3'

vars:
  IMAGE_TAG: 'latest'

tasks:
  default:
    vars:
      works: '{{.IMAGE_TAG}}'
      does:
        map:
          not:
            work: '{{.IMAGE_TAG}}'
    cmds:
      - 'echo {{.works}}'
      - 'echo {{.does.not.work}}'

Running the above Taskfile in v3.35.0 or lower will result in the following:

> TASK_X_ANY_VARIABLES=2 task
task: [default] echo latest
latest
task: [default] echo {{.IMAGE_TAG}}
{{.IMAGE_TAG}}

Not that the .does.not.work variable contains a template, but it is not parsed. After this PR the result will work as expected:

> TASK_X_ANY_VARIABLES=2 task
task: [default] echo latest
latest
task: [default] echo latest
latest

This is possible because any variable can now be passed into the templater.Replace(...) function and it will automatically traverse the value and find all strings that need replacing, whether they are raw strings, or nested strings inside a map/slice.

It's worth mentioning that I have not actually removed the old templater.Template struct. It is now called templater.Cache (as this is its primary use) and it is passed in as a regular function parameter instead. It's possible that we could make changes to this too, but I figured I'd leave that for another day rather than trying to bundle a bunch of error handling changes in the same PR.

[andreynering]

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