Add support for changelog entry markdown files (#3763)

## Motivation and Context
[RFC: File-per-change
changelog](https://smithy-lang.github.io/smithy-rs/design/rfcs/rfc0042_file_per_change_changelog.html#rfc-file-per-change-changelog)

## Description
This PR is the first of the two implementing the proposal outlined in
the RFC, and it focuses on the initial 4 bullets in the [Changes
checklist](https://smithy-lang.github.io/smithy-rs/design/rfcs/rfc0042_file_per_change_changelog.html#changes-checklist).
Crucially, this update does not modify the workflow for developers
regarding changelog entries (editing `CHANGELOG.next.toml` remains
necessary) or impact the `smithy-rs` CI process and our release
pipeline.

The PR introduces support for deserializing the following Markdown front
matter in the YAML format described in the RFC, e.g.
```
---
# Adding `aws-sdk-rust` here duplicates this entry into the SDK changelog.
applies_to: ["client", "server", "aws-sdk-rust"]
authors: ["author1", "author2"]
references: ["smithy-rs#1234", "aws-sdk-rust#1234"]
# The previous `meta` section is broken up into its constituents:
breaking: false
# This replaces "tada":
new_feature: false
bug_fix: false
---

Some message for the change.
```
These changelog entry Markdown files will be saved in the
`smithy-rs/.changelog` directory:
```
┌───────────┐                  
│ smithy-rs │                  
└─────┬─────┘                  
      │                        
      │    ┌───────────┐       
      ├────┤.changelog │       
           └─────┬─────┘       
                 │             
                 ├─────test1.md  
                 │ 
                 └─────test2.md 
```
For a concrete example, see a test
`end_to_end_changelog_entry_markdown_files` that directly converts from
`end_to_end_changelog` and uses Markdown files (the expected test
results remain the same).

The next PR is expected to
- add new subcommands to `changelogger` to a) create a Markdown file and
b) preview changelog entries pending to be released
- port existing `CHANGELOG.next.toml` at that time to individual
Markdown files
- update `sdk-lints` to disallow the existence of `CHANGELOG.next.toml`
- pass a directory path for `smithy-rs/.changelog` to `--source` (one
for
[split](https://github.com/smithy-lang/smithy-rs/blob/dc970b37386b155eced5d41262ce0a7fc4a34a91/tools/ci-scripts/generate-smithy-rs-release#L22)
and one for
[render](https://github.com/smithy-lang/smithy-rs/blob/dc970b37386b155eced5d41262ce0a7fc4a34a91/tools/ci-scripts/generate-smithy-rs-release#L22))
and to
[--source-to-truncate](https://github.com/smithy-lang/smithy-rs/blob/dc970b37386b155eced5d41262ce0a7fc4a34a91/tools/ci-scripts/generate-smithy-rs-release#L23C1-L23C47)

(the 4th bullet effectively does the cutover to the new changelog mode)

## Testing
- Added `parse_markdown` and `end_to_end_changelog_entry_markdown_files`
for testing new changelog format (the rest of the tests showing up in
the diff are moved from different places)
- Confirmed via [a release
dry-run](https://github.com/smithy-lang/smithy-rs/actions/runs/9947165056)
that this PR does not break the existing `smithy-rs` release process
- Confirmed that this PR does not break the use of `changelogger` in our
internal release process

----

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