feat(text): stabilize truncate()

[tomas-zijdemans]

Promotes truncate() to stable after five months unstable with zero bug reports. The new specifier is @std/text/truncate and it's now exported from mod.ts.

The diff is a rename plus bookkeeping: unstable_truncate.ts to truncate.ts (test file too), @experimental tag dropped, JSDoc import specifiers updated, deno.json export swapped. No logic changed. Behavior is identical to what's been on JSR since March (#7052).

One open question from the original request (#6456) is word and grapheme-aware truncation, which this API doesn't do. That's not a blocker: it lands later as an additive boundary option defaulting to today's behavior. What stabilization freezes is the defaults, so those are what to review hardest: UTF-16 code-unit measurement (matches every JS string builtin), no whitespace trimming before the suffix, and … as the default suffix.

Tested: 19 unit tests and 4 doc examples pass, lint and fmt green.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.03%. Comparing base (ca58f94) to head (04771a8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7295      +/-   ##
==========================================
- Coverage   95.03%   95.03%   -0.01%     
==========================================
  Files         617      618       +1     
  Lines       51637    51641       +4     
  Branches     9359     9359              
==========================================
+ Hits        49075    49078       +3     
  Misses       2021     2021              
- Partials      541      542       +1     

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