refactor: Centralize CONTEXT style

[epage]

What does this PR try to resolve?

This is similar to #16124 in making it easier for others to see what styles cargo uses and copy them.

How to test and review this PR?

[rustbot]

r? @weihanglo

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[weihanglo]

see rust-lang/rust#139070

 ---- package_features::other_member_from_current stdout ----
running `D:\a\cargo\cargo\target\debug\cargo.exe run -p bar --features f1`
running `D:\a\cargo\cargo\target\debug\cargo.exe run -p bar --features f1,f2`
running `D:\a\cargo\cargo\target\debug\cargo.exe run -p bar --features bar/f1`
running `D:\a\cargo\cargo\target\debug\cargo.exe run -p bar --features f1`

thread 'package_features::other_member_from_current' (7188) panicked at tests\testsuite\package_features.rs:362:10:

test failed running `D:\a\cargo\cargo\target\debug\cargo.exe run -p bar --features f1`
error: process exited with code 101 (expected 0)
--- stdout

--- stderr
   Compiling bar v0.1.0 (D:\a\cargo\cargo\target\tmp\cit\t2603\foo\bar)

thread 'main' (5588) has overflowed its stack
error: could not compile `bar` (lib)