IDLE: Wrong highlighting when previous line ends with backslash

Bug report

Bug description:

Code like this

str. \
    format

should be coloured the same way as

str.format

Right now IDLE doesn't respect the slash before the newline and thinks that every new line is a new statement.

Similarly in:

x = match if match else \
                   match

The last match should be coloured in like a variable and not a keyword.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-140335
• gh-140337

从 IDLE 的语法高亮行为入手，重现涉及换行前反斜杠的两个示例。验证续行表达式的着色方式与等效的单行表达式相同，包括最后的匹配变量；已链接的 PRs gh-140335 和 gh-140337 表明这项工作已经在进行中。