More expansions in template repositories

[jolheiser]

Also fixes a regression from #9489 where there is a nil error.
It was not backported (and is correct in 1.11)

This PR adds some extra options for expansions in template repositories.
As well, I decided to move the docs into its own since it's already the entire page.

Adds a new module xstrings (MIT) to avoid maintaining edge-cases for string conversions to various -case
xstrings is actually the repo that com takes its ToSnakeCase func from anyways.

[codecov-io]

Codecov Report

Merging #10021 into master will increase coverage by <.01%.
The diff coverage is 59.31%.

@@            Coverage Diff             @@
##           master   #10021      +/-   ##
==========================================
+ Coverage   42.26%   42.26%   +<.01%     
==========================================
  Files         610      611       +1     
  Lines       80389    80396       +7     
==========================================
+ Hits        33974    33977       +3     
- Misses      42235    42240       +5     
+ Partials     4180     4179       -1

Impacted Files	Coverage Δ
services/pull/review.go	0% <0%> (ø)	⬆️
routers/repo/commit.go	29.66% <0%> (ø)	⬆️
modules/migrations/gitea.go	6.14% <0%> (-0.02%)	⬇️
services/gitdiff/gitdiff.go	67.62% <100%> (+1.36%)	⬆️
models/pull.go	43.24% <100%> (+0.33%)	⬆️
modules/git/diff.go	62.03% <62.03%> (ø)
services/pull/temp_repo.go	31.62% <0%> (-2.57%)	⬇️
modules/queue/workerpool.go	39.91% <0%> (-1.29%)	⬇️
modules/log/event.go	64.61% <0%> (-1.03%)	⬇️
... and 3 more

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[zeripath]

make lg-tm work