chore(renovate): ignore bundler-tests directory

[pichlermarc]

Which problem is this PR solving?

Ignores the bundler-tests directory as a good chunk of the tools we test against are outdated on purpose.

Idea is to prevent PRs like:

• fix(deps): update react monorepo to v19 (major) - autoclosed #6312
• fix(deps): update dependency next to v16 - autoclosed #6311
• chore(deps): update dependency html-webpack-plugin to v5 - autoclosed #6247

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.46%. Comparing base (a342d4e) to head (014aea5).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6314      +/-   ##
==========================================
+ Coverage   95.41%   95.46%   +0.05%     
==========================================
  Files         317      317              
  Lines        9597     9599       +2     
  Branches     2220     2218       -2     
==========================================
+ Hits         9157     9164       +7     
+ Misses        440      435       -5     

see 3 files with indirect coverage changes

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[trentm]

Apparently renovate.json is parsed as JSONC (https://docs.renovatebot.com/configuration-options/), so adding comments is possible if we want to describe intent later on. This is not meant to slow down this PR.