Add 'type' to LandingPage; fix Catalog and Collection type capitalization

[lossyrob]

A LandingPage is a Catalog, and so should have the type field.

Also, the spec says the C of Collection and Catalog should be capitalized.

[codecov-commenter]

Codecov Report

Merging #96 (951aa92) into master (521ecd8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #96   +/-   ##
=======================================
  Coverage   96.18%   96.19%           
=======================================
  Files          20       20           
  Lines         472      473    +1     
=======================================
+ Hits          454      455    +1     
  Misses         18       18           

Flag	Coverage Δ
unittests	96.19% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
stac_pydantic/api/landing.py	100.00% <100.00%> (ø)
stac_pydantic/catalog.py	100.00% <100.00%> (ø)
stac_pydantic/collection.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 521ecd8...951aa92. Read the comment docs.