Use install.libs.R instead of configure

[wlandau]

Prework

• [x] I understand and agree to the Contributor Code of Conduct.
• [x] I have already submitted a GitHub Discussion topic to discuss my idea with the maintainer.

Related GitHub issues and pull requests

• Ref: #12 (also seems to fix #1 and #9 on my end)

Summary

• Use install.libs.R for both CmdStan in instantiate and Stan model compilation in packages (#1, #9, #12).
• Stan models should now be in src/stan/ instead of inst/stan/ (#12). inst/stan/ should still work for the next few versions, but it is now deprecated. When a modeling package is installed, its install.libs.R moves src/stan/ to bin/stan/ and then compiles models inside bin/stan/.
• Disable staged installation for instantiate itself to help the hard-coded paths in CmdStan work as expected in "internal" installation.

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (f910e35) 100.00% compared to head (482fb36) 98.29%.

Files	Patch %	Lines
src/cmdstan.c	83.33%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #13 +/- ## =========================================== - Coverage 100.00% 98.29% -1.71% =========================================== Files 14 14 Lines 272 294 +22 =========================================== + Hits 272 289 +17 - Misses 0 5 +5 ```

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