Allow extensions to have multiple filters at different phases of the lifecycle

[multimeric]

Description

Quarto 1.4 introduced lifecycle phase targeting. Unfortunately this still isn't available for extensions.

For example, with a _extension.yml that contains this:

contributes:
  filters:
  - at: pre-ast
    path: filter1.lua
  - at: post-quarto
    path: filter2.lua
  - at: post-render
    path: filter3.lua

In file _extensions/foo/_extension.yml
(line 7, column 5 through line 8, column 22) Array entry 1 with value at: pre-ast
  path: filter1.lua failed to be a string.
6:   filters:
7:   - at: pre-ast
      ~~~~~~~~~~~~
8:     path: filter1.lua
  ~~~~~~~~~~~~~~~~~~~~~~
9:   - at: post-quarto
✖ The value

However, it is desirable for extensions to be able to specify this, as they might need to hook into the lifecycle at multiple phases.

[multimeric]

Oops, seems to have already been discussed in #11560 (comment).