Add new subcommands to `changelogger` for supporting file-per-change changelog (#3771)

## Motivation and Context
Adds new subcommands `--new` and `--ls` to `changelogger`

## Description
This is part 2 in a series of PRs supporting [file-per-change
changelog](https://smithy-lang.github.io/smithy-rs/design/rfcs/rfc0042_file_per_change_changelog.html).
This PR just adds utilities for humans and does NOT impact the current
dev workflow, smithy-rs CI, or our release pipeline.

#### A subcommand `--new`
We can use this subcommand when creating a new changelog entry markdown
file. An example usage:
```
$ changelogger new -t client -t aws-sdk-rust -r smithy-rs#1234 -a someone --bug-fix -m "Some changelog" \
# for long flags, replace -t with --applies-to, -r with --references, -a with --authors, and -m with --message \
# also remember to escape with \ when including backticks in the message at command line, e.g. \`pub\`

The following changelog entry has been written to "/Users/ysaito1001/src/smithy-rs/.changelog/5745197.md":
---
applies_to:
- aws-sdk-rust
- client
authors:
- someone
references:
- smithy-rs#1234
breaking: false
new_feature: false
bug_fix: true
---
Some changelog
```
The following CLI arguments are "logically" required
- `--applies-to`
- `--ref`
- `--author`
- `--message`

If any of the above is not passed a value at command line, then an
editor is opened for further edit (which editor to open can be
configured per the [edit crate](https://docs.rs/edit/0.1.5/edit/)).

Note that the YAML syntax above is different from the single line array
syntax [proposed in the
RFC](https://smithy-lang.github.io/smithy-rs/design/rfcs/rfc0042_file_per_change_changelog.html#the-proposed-developer-experience).
This is due to [a
limitation](https://github.com/dtolnay/serde-yaml/issues/355) in
`serde-yaml` (which has been archived unfortunately), but the multi-line
values are still a valid YAML syntax and, most importantly, rendering
changelong entries will continue working. For now, I won't post-process
from the multi-line values syntax to the single line array syntax. One
can work around this by handwriting a changelog entry Markdown file in
`smithy-rs/.changelog` without using this subcommand.

#### A subcommand `--ls`
This subcommand will render a preview of changelog entries stored in
`smithy-rs/.changelog` and print it to the standard output. Example
usages (using the entry created at 5745197.md above):
```
$ changelogger ls -c smithy-rs \
# for long flag, replace -c with --change-set

Next changelog preview for smithy-rs
=====================================
**New this release:**
- 🐛 (client, [smithy-rs#1234](https://github.com/smithy-lang/smithy-rs/issues/1234), @someone) Some changelog

**Contributors**
Thank you for your contributions! ❤️
- @someone ([smithy-rs#1234](https://github.com/smithy-lang/smithy-rs/issues/1234))
```
```
$ changelogger ls -c aws-sdk \
# for long flag, replace -c with --change-set

Next changelog preview for aws-sdk-rust
========================================
**New this release:**
- 🐛 ([smithy-rs#1234](https://github.com/smithy-lang/smithy-rs/issues/1234), @someone) Some changelog

**Contributors**
Thank you for your contributions! ❤️
- @someone ([smithy-rs#1234](https://github.com/smithy-lang/smithy-rs/issues/1234))
```


## Testing
- Existing tests in CI
- Basic unit tests for subcommands

----

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