Support fetch priority with import maps

[erikt9]

Fixes #2478

Invoke like:
<script type="system-js-importmap" src="https://import_map_url" fetchpriority="high">

[github-actions]

File size impact

Merging import_map_fetch_priority into main will impact 3 files in 2 groups.

browser (2/2)

File	raw	gzip	brotli	Event
dist/s.min.js	+25 (7,907)	+10 (3,133)	+12 (2,845)	modified
dist/system.min.js	+25 (12,256)	+13 (4,744)	+10 (4,272)	modified
Total size impact	+50 (20,163)	+23 (7,877)	+22 (7,117)

node (1/1)

File	raw	gzip	brotli	Event
dist/system-node.cjs	+32 (521,949)	+11 (126,136)	+28 (84,951)	modified
Total size impact	+32 (521,949)	+11 (126,136)	+28 (84,951)

extras (0/8)

No impact on files in extras group.

_{Generated by file size impact during size-impact#7417913891}

[guybedford]

Thanks, looks great!