Refactor BAM record class to avoid lower casing all tag names

[cmdcolin]

Lower case tags and upper case tags are unique namespaces in the SAM spec

For legacy reasons, the @gmod/bam followed basically what JBrowse 1 did but that included lower casing everything, and making all variable access go through a "feature.get('somestring')" method for both core data like feature.get('start') and and arbitrary tags like feature.get('XS')

this changes the code to be more like the CRAM codebase, which uses

for core attribytes: feature.start (where feature.start is a "getter" on the record class)

and for tags: feature.tags.XS (where feature.tags is a getter for an object for a Record<string,unknown>

[cmdcolin]

demonstration of usage in jbrowse-components
https://github.com/GMOD/jbrowse-components/compare/new_bam_parser?expand=1