Add setheader!, appendheader! and removeheader! aliases

[quinnj]

Closes #1277.

Problem

setheader, appendheader and removeheader mutate their first argument but carry no !, so readers expect them to return copies. The thread on #1277 settled on adding the ! spellings without deprecating the historical names, since those also existed in 1.x.

Change

• setheader!, appendheader! and removeheader! are defined as const aliases of the existing functions, so each pair is the same function object: every existing method (both the Headers and the Request/Response forms) is reachable under either name, and a package extending one name extends the other.
• The historical names are not deprecated. Their docstrings now point at the ! spelling as the preferred form.
• All three new names are declared public alongside the old ones and are listed on the core API docs page.
• CHANGELOG entry added.

Tests

Mutating-name header aliases (#1277) in test/http_core_tests.jl checks identity of the aliases, the Headers and message-level forms (including the comma-merge behavior of appendheader! and case-insensitive removeheader!), and that the names are public on Julia 1.11+.

test/http_core_tests.jl passes in full locally on Julia 1.12.6.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.51%. Comparing base (d6f049a) to head (d2a4f1c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1359      +/-   ##
==========================================
+ Coverage   89.42%   89.51%   +0.08%     
==========================================
  Files          31       31              
  Lines       12594    12594              
==========================================
+ Hits        11262    11273      +11     
+ Misses       1332     1321      -11     

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