LibSyntax is the only remaining user of UTF-32 in the code base. Let's
use UTF-8 here.
Bonus: The tests added here for non-ASCII sources actually used to
crash the old UTF-32 implementation.
The RehighlightState designated initializer used `.position = {}`
which invokes TextPosition's default constructor, initializing line
and column to 0xFFFFFFFF (the "invalid" sentinel). This overrode
the struct's default member initializer of { 0, 0 }.
When advance_position() processed the first newline, it incremented
0xFFFFFFFF to 0x100000000, producing line numbers in the billions.
These bogus positions propagated into folding regions, causing an
out-of-bounds crash in Document::set_folding_regions() when viewing
page source on pages with <script> blocks.
Fix by explicitly initializing position to { 0, 0 }.
Fixes#8529.
