Refactor/rename postprocess_steno_user → post_process_steno_user

[precondition]

Description

Small refactor to bring the post processing steno callback function in line with every other post processing QMK function.

❯ git checkout develop
Switched to branch 'develop'
Your branch is up to date with 'upstream/develop'.

~/qmk_firmware on develop *3 ⨤4                                                                                                                                                                       
❯ which grepqmk
grepqmk: aliased to fd -t f -E docs -E keyboards -E layouts -E users -E lib -X grep -n --color

~/qmk_firmware on develop *3 ⨤4                                                                                                                                                                       
❯ grepqmk "post_\?process"
quantum/action.c:223:__attribute__((weak)) void post_process_record_quantum(keyrecord_t *record) {}
quantum/action.c:269:    post_process_record_quantum(record);
quantum/action.h:93:void post_process_record_quantum(keyrecord_t *record);
quantum/process_keycode/process_steno.c:151:__attribute__((weak)) bool postprocess_steno_user(uint16_t keycode, keyrecord_t *record, steno_mode_t mode, uint8_t chord[MAX_STROKE_SIZE], int8_t n_pressed_keys) {
quantum/process_keycode/process_steno.c:212:                if (!postprocess_steno_user(keycode, record, mode, chord, n_pressed_keys)) {
quantum/process_keycode/process_steno.c:217:                if (!postprocess_steno_user(keycode, record, mode, chord, n_pressed_keys)) {
quantum/quantum.c:129:__attribute__((weak)) void post_process_record_kb(uint16_t keycode, keyrecord_t *record) {
quantum/quantum.c:130:    post_process_record_user(keycode, record);
quantum/quantum.c:133:__attribute__((weak)) void post_process_record_user(uint16_t keycode, keyrecord_t *record) {}
quantum/quantum.c:216:void post_process_record_quantum(keyrecord_t *record) {
quantum/quantum.c:218:    post_process_record_kb(keycode, record);
quantum/quantum.h:255:void     post_process_record_kb(uint16_t keycode, keyrecord_t *record);
quantum/quantum.h:256:void     post_process_record_user(uint16_t keycode, keyrecord_t *record);

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).