BREAKING(dotenv): remove deprecated loaders

[tomas-zijdemans]

Removes load(), loadSync(), LoadOptions and the @std/dotenv/load export, as agreed in #7246. parse() and stringify() stay, and the migration notes in mod.ts stay too.

Version. The @deprecated tags say "removed in 0.227.0", but that assumed the deprecation would ship as 0.226.0. It shipped as 0.225.8, because deprecation titles bump the patch. On a 0.x package a BREAKING title bumps the minor, so this lands as 0.226.0. Nobody gets surprised by it: ^0.225.8 doesn't match 0.226.0. If you'd rather keep the notice literal, the release PR can set 0.227.0 instead. Fine by me either way.

Tests. mod_test.ts held the only tests for parse() reading from the process env, since they went through loadSync(). I moved three of those cases to parse_test.ts:

• an env var set to "" expands to ""
• no env access is needed when nothing expands from the process env
• expanding without env access throws NotCapable

Tested with env -u NO_COLOR deno task ok.

I used Claude Code to help investigate and write this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.04%. Comparing base (f834d02) to head (b6f83e4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7335   +/-   ##
=======================================
  Coverage   95.04%   95.04%           
=======================================
  Files         619      618    -1     
  Lines       52012    51960   -52     
  Branches     9450     9438   -12     
=======================================
- Hits        49434    49385   -49     
+ Misses       2031     2029    -2     
+ Partials      547      546    -1     

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