[SCHEMA] Define the types present in the schema in a new YAML file

[tsalo]

Closes None, but it might be related to #884?

Changes proposed:

• Add a new schema/objects/types.yaml file with definitions of the valid "object types" (e.g., suffixes, metadata fields) within the schema. This gives each a name and description.
• Additionally, try to define valid fields for different object types.
  • For example, columns and metadata fields may have a number of fields describing the value types, such as "boolean" or "number", while associated data only have names and descriptions.

To do:

• I still need to figure out how to actually validate the schema based on the new rules.

[codecov]

Codecov Report

Merging #1048 (98b53e3) into master (73d9968) will increase coverage by 4.13%.
The diff coverage is 100.00%.

❗ Current head 98b53e3 differs from pull request most recent head dfb8767. Consider uploading reports for the commit dfb8767 to get more accurate results

@@            Coverage Diff             @@
##           master    #1048      +/-   ##
==========================================
+ Coverage   88.09%   92.22%   +4.13%     
==========================================
  Files           6       10       +4     
  Lines         974     1145     +171     
==========================================
+ Hits          858     1056     +198     
+ Misses        116       89      -27     

Impacted Files	Coverage Δ
tools/schemacode/bidsschematools/schema.py	82.95% <100.00%> (+2.26%)	⬆️
tools/schemacode/bidsschematools/utils.py	73.00% <0.00%> (-7.00%)	⬇️
tools/schemacode/bidsschematools/validator.py	92.98% <0.00%> (-1.56%)	⬇️
tools/schemacode/bidsschematools/render.py	93.04% <0.00%> (-0.09%)	⬇️
tools/schemacode/bidsschematools/__init__.py	100.00% <0.00%> (ø)
tools/schemacode/bidsschematools/__main__.py
...schemacode/bidsschematools/tests/test_validator.py	100.00% <0.00%> (ø)
tools/schemacode/bidsschematools/tests/conftest.py	91.66% <0.00%> (ø)
...ls/schemacode/bidsschematools/tests/test_schema.py	100.00% <0.00%> (ø)
...ls/schemacode/bidsschematools/tests/test_render.py	100.00% <0.00%> (ø)
... and 2 more

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[tsalo]

This is the most extensive "definition," so I think it's the best one to focus on as we determine if this structure is any good.

[effigies]

Feels out of scope for the current sprint. We will not likely parse this in any first implementations, but might be worth revisiting once the schema is finalized.

[tsalo]

Should I define these and other fields in this file as well?