fix(fluent-syntax): Count indented placeables toward common indent

[chiliec]

Fixes #286.

For a block pattern whose line starts with an indented placeable:

key12 =
    { "." }
        four

the parser skipped the placeable line's leading spaces and then dropped the resulting empty text slice. That line never counted toward the common indent, so only four's line did (8). All 8 spaces were stripped and the message parsed the same as if four had been indented by 4.

The fix: when a line-start text slice ends at a placeable, its indent now counts toward the common indent and it is kept as a text element, like the Indent token in fluent.js's getPattern/dedent. Elements that are empty after dedenting are dropped, so placeable lines at the common indent produce the same AST as before.

Examples:

• { "." } at 4 spaces, four at 8 → " four" (was "four")
• a at 4, { x } at 8 → "a\n", " ", { x } (the placeable's extra indent used to be lost)

Tests

• Added the case from the issue to tests/fixtures/normalized/messages.ftl. Without the parser change roundtrip_normalized_fixtures fails (the line is reserialized as four). With it, the test passes.
• cargo test --workspace: all pass
• cargo fmt --all -- --check and cargo clippy -p fluent-syntax --all-targets: clean

crlf.ftl has to stay in IGNORE_LIST. Its roundtrip still fails for an unrelated reason: the CRLF Value 02 / \n text-element split differs from the reparsed LF output.