* feat: Add configs for `reply-with` and `forward-with` commands
Config extended from:
(Old)
```toml
[message.composer.mml]
command = "mml compose"
```
, where `compose-with`, `reply-with`, and `forward-with`
all share the same composer command, to:
(New)
```toml
[message.composer.mml]
default = true
compose-command = "mml compose"
reply-command = "mml reply"
forward-command = "mml forward"
```
* docs: ComposerConfig
* refactor(account): simplify composer resolution with `get_composer`
- Implement `Account::get_composer`, and `Account::get_reader`
to fetch config by name or default.
- Remove the redundant `resolve_composer`, `default_composer`
`resolve_reader`, and `default_reader` helpers.
- Simplify the configuration retrieval architecture.
* refactor: update composer and reader config to use std::process::Command
- Remove `_command` suffix from `compose`, `reply`,
and `forward` configuration fields.
- Replace composer and reader config command type
from `String` to `std::process::Command`.
- Remove `Clone` derives from `Config`, `Account`, and
related structs due to `Command` type limitations.
Refs: #687
Bowen Ho