[rust-compiler] Emit loc.column/index as UTF-16 code units in SWC frontend

[poteto]

After the cluster-1 BytePos shift, ConvertCtx::position() emitted
loc.column and loc.index as 0-based UTF-8 byte offsets. Babel emits
them as 0-based UTF-16 code unit offsets (matching JS string indexing).
For files containing any character above U+FFFF (e.g. an emoji like
🔴 U+1F534), the two diverge by +2 per such character because the
char is 4 bytes in UTF-8 but 2 code units in UTF-16.

Precompute a utf16_offsets: Vec<u32> table in ConvertCtx::new
that maps each source byte index to its 0-based UTF-16 code unit
offset. position() then looks up index directly and computes
column as index - utf16_index_of_line_start. O(1) per call; the
table costs ~4× the source length in memory, which is bounded for
fixture/file inputs.

Considered an alternative that walks the source line on each
position() call to count UTF-16 code units. More memory-frugal but
O(line length) per call. The precomputed table wins on O(1) lookup
and the per-call cost matters because position() is invoked on
every node, comment, and reference in the converter.

Clamp the byte index in position() to the sentinel at
utf16_offsets.len() - 1. Synthetic spans (e.g. compiler-generated
imports given BytePos(1)) can point past EOF in degenerate cases;
clamping avoids a panic.

Line numbers stay 1-based and the binary-search remains keyed on
byte offsets, since the underlying line_offsets table is byte-based.

Fixes 4 e2e parity fixtures (3 targeted + 1 latent):

• effect-derived-computations/invalid-derived-computation-in-effect.js
• error.invalid-derived-computation-in-effect.js
• fbt/error.todo-multiple-fbt-plural.tsx
• (one additional latent fixture passes for free)

Test plan:

• bash compiler/scripts/test-e2e.sh --variant swc:
  Before: Total 1770/1795
  After: Total 1774/1795 (4 fixed)
• bash compiler/scripts/test-e2e.sh --variant babel: 1788/1795 (unchanged)
• bash compiler/scripts/test-e2e.sh --variant oxc: 1702/1795 (unchanged)
• cargo test --workspace: 56 passed, 0 failed