Use .jlt instead of .jl for template files

[fredrikekre]

This patch changes the file extension of Julia template files from .jl to .jlt. The motivation for this is tooling that assume these are (valid) julia source files based on the .jl extension.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.33%. Comparing base (a007c9e) to head (813835a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #481   +/-   ##
=======================================
  Coverage   94.33%   94.33%           
=======================================
  Files          24       24           
  Lines         742      742           
=======================================
  Hits          700      700           
  Misses         42       42           

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[oxinabox]

I think this makes sense.
Most syntax highlighting tools etc (that are ok with broken code) still are not that useful on the template files.
And the language can always be set manually.

Do you have merge rights?

[oxinabox]

This CI error is real:
https://github.com/JuliaCI/PkgTemplates.jl/actions/runs/10521345275/job/29151850911?pr=481#step:7:137

(One annoying thing about this repo is we do do different tests on different versions of julia)

[fredrikekre]

This CI error is real:

Fixed, didn't show up locally (on the same Julia version).

Do you have merge rights?

Looks like it.

[oxinabox]

LGTM
Probably bump as a nonbreaking change so you can tag a release after you merge