fix: untrack largeish file (#7005)

This shouldn't be tracked in git, as it's meant for each developer machine's to keep state
for incremental compilation speed.

This was also ending up in the published package, where it accounted for ~24% of the
tarball (320 out of 1330 KB).

.gitignore

@@ -33,3 +33,6 @@ site/src/**/*.md
 _test_out/**
 *.crt
 *.key
+
+# Used in local dev by tsc: https://www.typescriptlang.org/tsconfig/#tsBuildInfoFile
+tsconfig.tsbuildinfo