add dynamic import meta placeholder

[wenerme]

import assert will going to make into stage 4, add a placeholder for future impl, this may help the module-types extra (will add to core), a placeholder mean the bundler can pass the assertion, but currently only for dynamic import.

for import 'xyz' assert {type:'xyz'}, this maybe harder to decided the how api look.

[github-actions]

File size impact

Merging import-meta into main will impact 3 files in 2 groups.

browser (2/2)

File	raw	gzip	brotli	Event
dist/s.min.js	+63 (7,839)	+42 (3,111)	+33 (2,817)	modified
dist/system.min.js	+63 (12,173)	+39 (4,707)	+31 (4,229)	modified
Total size impact	+126 (20,012)	+81 (7,818)	+64 (7,046)

node (1/1)

File	raw	gzip	brotli	Event
dist/system-node.cjs	+254 (519,076)	+97 (125,349)	-21 (84,148)	modified
Total size impact	+254 (519,076)	+97 (125,349)	-21 (84,148)

extras (0/8)

No impact on files in extras group.

_{Generated by file size impact during size-impact#3181281964}