feat(schema): Add meta.templates and allow for composing multiple references

[effigies]

This is pulled out of #1714, which needs a way of constructing file rules by composing raw rules with rules adapted for atlas datasets. Since atlas datasets are just derivatives datasets, we can do much of the work in advance.

[effigies]

@oesteban Pinging you because this is adapted from your work in BEP38. I'll also make a PR against BEP38 with the relevant changes.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 82.85%. Comparing base (dd1e5d2) to head (d66c823). :warning: Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
tools/schemacode/src/bidsschematools/schema.py	94.73%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2140      +/-   ##
==========================================
+ Coverage   82.71%   82.85%   +0.13%     
==========================================
  Files          20       20              
  Lines        1608     1621      +13     
==========================================
+ Hits         1330     1343      +13     
  Misses        278      278              

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[effigies]

Had to rebase to account for #1440 and #2100. Added documentation on null deletion.

[effigies]

This schema induces new errors in the examples. Will need to dig deeper before merging. It would be good to add a CI job to run draft schemas on the examples.

[effigies]

Might be easiest to review with https://ignoreorderdiff.com/.