Add \ to emphasis post chars to allow immediate explicit line break

Post chars are defined in (nth 1 org-emphasis-regexp-components) in emacs org.
When I initially adapted the list of chars for go, I failed to check how it's
actually used (further down in org.el):
  (string-match (concat "[" (nth 1 erc) "\n]") (char-to-string (char-after (point))))

Due to the surrounding [] the `\\` in
  '("-[:space:]('\"{" "-[:space:].,:!?;'\")}\\[" "[:space:]" "." 1)

is actually a literal backslash, not an escape of the opening bracket I
guess. I'm not in the mood for thinking any harder about this, so let's hope
this is right. yolo.

org/testdata/inline.org

@@ -5,6 +5,8 @@
 - /emphasis with a slash/inside/
 - /emphasis/ followed by raw text with slash /
 - *emphasis ending with a "difficult" multibyte character 习*
+- emphasis just before =explict line break=\\
+  =plus more emphasis=
 - ->/not an emphasis/<-
 - links with slashes do not become /emphasis/: [[https://somelinkshouldntrenderaccidentalemphasis.com]]/ /emphasis/
 - _underlined_ *bold*  =verbatim= ~code~ +strikethrough+