[Commands] Initial implementation of swift package add-setting command (#8532)

### Motivation:

Adds a way to programmatically insert new settings into a package
manifest.

Currently only some Swift settings are supported, namely:
`enable{Upcoming, Experimental}Feature`, `swiftLanguageMode` and
`strictMemorySafety`; but the command could be expanded to support more
Swift (C, C++, linker) settings in the future.

### Modifications:

- Add a new "package" sub-command named "add-setting" that accepts a
target and a list of `--swift Name[=Value]?` pairs for each new swift
setting to add. Each setting would make sure that manifest is new enough
to support it.
- Add new manifest refactoring action - AddSwiftSetting that would add
`swiftSettings:` to a target or modify the existing one.
- Expands existing way to check whether manifest is too old to support
custom versions. This is doe to make sure that users don't add settings
that are not supported by the manifest tools version i.e.
`swiftLanguageMode` was introduced in 6.0 tools.

### Result:

A new `swift package add-setting --target <name> [--swift
Name[=Value]?]+` command that allows users to programmatically add new
settings to package manifests.

Author: xedin