fix: handle leading tilde on else if chain tags

[sunng87]

Fixes #766.

Summary

{{~else if cond}} panicked during template compilation:

thread 'main' panicked at src/template.rs:389:22:
internal error: entered unreachable code: leading_tilde_to_omit_whitespace

Root cause

The grammar places the optional leading tilde of invert_chain_tag
before invert_tag_item:

invert_chain_tag = { !escape ~ "{{" ~ leading_tilde_to_omit_whitespace? ~ invert_tag_item
                     ~ exp_line ~ trailing_tilde_to_omit_whitespace? ~ "}}" }

But the compile code called parse_name (to consume invert_tag_item)
before parse_expression, so when a leading ~ was present it was
handed to parse_name, which only expects identifier /
invert_tag_item / reference / subexpression — hence the
unreachable! arm.

The plain {{~else}} (invert_tag) case already worked, because it
skips parse_name and goes straight to parse_expression, which
handles the leading tilde itself. Only invert_chain_tag with a
leading tilde was broken (trailing tildes worked, since
parse_expression's loop consumes them).

Fix

Consume the optional leading tilde before parse_name for
invert_chain_tag, and fold its omit_pre_ws semantics into the
parsed expression so preceding whitespace is stripped consistently
with {{~else}}.

let mut invert_omit_pre_ws = false;
if rule == Rule::invert_chain_tag {
    if it.peek().unwrap().as_rule() == Rule::leading_tilde_to_omit_whitespace {
        invert_omit_pre_ws = true;
        it.next();
    }
    let _ = Template::parse_name(source, &mut it, span.end())?;
}
let mut exp = Template::parse_expression(source, it.by_ref(), span.end())?;
if invert_omit_pre_ws {
    exp.omit_pre_ws = true;
}

Tests

• src/grammar.rs: grammar acceptance cases for {{~else if foo}},
  {{else if foo~}}, {{~else if foo~}}.
• src/template.rs: unit test reproducing the original panic.
• tests/whitespace.rs: rendering tests for leading / trailing / both
  tildes and the falsy fallback, confirming behaviour matches {{~else}}.

All existing tests, cargo fmt --check, and cargo clippy --tests pass.

[coveralls]

coverage: 85.052% (+0.05%) from 85.007% — fix/else-if-leading-tilde into master