request: Support YAML anchors to in Melos YAML schema to limit repetition

### Is there an existing feature request for this?

- [X] I have searched the existing issues.

### Command

_No response_

### Description

Suppose `melos.yaml` to contain:
```yaml
scripts:
foo:
run: echo foo
exec:
concurrency: 1
orderDependents: true

bar:
run: echo bar
exec:
concurrency: 1
orderDependents: true

baz:
run: echo baz
exec:
concurrency: 1
orderDependents: true
```
As you can see, the `exec` configuration is repeated over and over. YAML supports anchors and aliases, so we can instead write this:
```yaml
x-exec: &execConfig
concurrency: 1
orderDependents: true

scripts:
foo:
run: echo foo
exec: *execConfig

bar:
run: echo bar
exec: *execConfig

baz:
run: echo baz
exec: *execConfig
```
This already works and nothing needs to be done, this YAML feature is supported by the Dart YAML parser.

The only 'issue' is that if the Melos plugin is installed, which turns on Melos YAML schema validiation (or without the plugin, validation can also be turned on) this causes an error:

The name `x-exec` is arbitrary and can be whatever, and it is not expected, hence the error, but this can be fixed. As an example, Docker Compose uses YAML as well for its config and it allows 'extension fields': https://docs.docker.com/compose/compose-file/compose-file-v3/#extension-fields (this is where the `x-` prefix comes from in my example). Here is how they do it: https://raw.githubusercontent.com/compose-spec/compose-spec/master/schema/compose-spec.json (search for x-), here more about 'pattern properties': https://json-schema.org/understanding-json-schema/reference/object.html#pattern-properties

Would it be possible to make it work for Melos as well?

### Reasoning

This would make the schema more flexible and ready for more advanced YAML features, which are already supported anyway.

### Additional context and comments

_No response_

Start by locating Melos's YAML schema validation and the schema definition used for melos.yaml, then reproduce the issue with the x-exec anchor example from the report. Done means arbitrary extension fields used for YAML anchors are accepted while regular Melos configuration remains validated; no specific files or tests are named in the issue.

Written by the indexing model from the issue text.