Custom marker mismatch between source project and target project when translating

There are cases where the source project defines custom markers that are not defined in the target project. Copying these markers into the draft text may cause issues with the target project if the custom marker is not defined in the target project. In some cases, the custom marker is not correctly recognized during parsing, and the draft text includes the opening marker but does not include the marker content nor closing marker.

As an example, the BHNSB translation defines an "extended footnote" marker ("\ef", "\ef*"). When books from the BHNSB are translated into a target language, the opening "\ef" marker is retrained in the draft that is created, while the content of the marker and the closing marker is removed. The BHNSB text for Ruth 1:22 has an extended footnote:

\v 17 Pale utakapofia hapo nitakufa nami, \q1 na papo hapo nitazikwa; \q1 Mwenyezi-Mungu anipe adhabu kali kama nikitenganishwa nawe \q1 isipokuwa tu kwa kifo.”\ef f \fr 1:16-17 \ft Aya hizi zimewekwa katika muundo wa utenzi au shairi. Muhimu katika utenzi huu ni tamko la Ruthu la kujiweka mmoja na jumuiya ya Waisraeli.\ef*

which gets translated as follows in the draft verse:

\v 17 Palu' pawaalu'a waa ngapaniwaa, no sikilwa. Nnu'ngu' anipei ukumu ngali' mana naabu'kana na wenga, ila kwa kiwo bai." \q1 \q1 \q1 \ef

(Note the opening marker ("\ef") with no marker content and closing marker.)

No files, tests, or entry points are named. Reproduce the BHNSB-to-target translation using the Ruth 1:22 example, then trace custom-marker parsing and draft generation; done means undefined target markers no longer leave an opening marker while dropping the marker content and closing marker.