feat(lint): Add conditional_no_std_attribute lint

[bushrat011899]

Objective

• Contributes to Lint crates that can be #![no_std] but aren't #1569

Details

This adds a lint which discourages usage of cfg_attr with no_std. This is usually bad because it changes the implicit prelude across the crate, making it trivial to fall into a trap where either you are warned for redundant imports, or fail to compile because of missing ones. The lint will suggest replacing:

#![cfg_attr(some_arbitrary_condition, ..., no_std, ...)]

With:

#![cfg_attr(some_arbitrary_condition, ...)]
#![no_std]
#[cfg(not(some_arbitrary_condition))]
extern crate std;

Notes

• No AI tooling of any kind was used during the creation of this PR.
• This suggestion can fail if the inner attribute is not the last one (since it would insert the extern crate statement between inner attributes). I have marked it as Applicability::MaybeIncorrect to reflect this.
• The cfg_attr statement will be removed entirely if the only attribute within it is no_std.
• The lint checks for MSRV 1.6.0 (the version #![no_std] was stabilised in), as crates supporting older versions of Rust may use cfg_attr to add no_std support without raising MSRV.

---

changelog: [conditional_no_std_attribute]: add new lint for conditional inclusion of #![no_std] attribute

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[github-actions]

Lintcheck changes for cc4638b

Lint	Added	Removed	Changed
clippy::conditional_no_std_attribute	47	0	0

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