map move_prev_sub_word_start and move_next_sub_word_end to H and L

[nik-rev]

Subword movement is extremely useful and it's a shame that it isn't mapped to anything. The PR that added it got over 50 positive reactions so it's clear people want it.

Currently, H and L are both not mapped to anything making them perfect candidates for mapping to subword movement.

• h moves one char left, and H moves to previous subword
• l moves one char right, and L moves to the next subword

It's a problem that these commands are not mapped to anything because it hurts discoverability and Helix is an editor that comes with things ready-to-go, so in my opinion it should be mapped by default.

here's the PR that adds it: #12028