Consolidate send_string implementations.

[tzarc]

Description

Consolidates handling of send_string et.al. such that the embedded commands using SS_QMK_PREFIX are consistently handled.

Tested on an AVR board, with string resident in RAM and PROGMEM. Tested with VIA and its use of dynamic_keymap.

Types of Changes

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

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).

[tzarc]

Hmm, not bad, saves 322 bytes for a VIA-enabled build:

% QMK_USERSPACE=$(realpath ..) ./util/size_regression.sh -s 0.27.0 -d HEAD kprepublic/bm16s:tzarc
Size:    26786, delta:   -322 -- d4ec060c39 Consolidate send_string implementations.
Size:    27108, delta:     +0 -- 7b83bb9d26 Allow running Docker container on MacOS (without in-container USB support) (#24412)
Size:    27108, delta:     +0 -- 7f1c236c75 Bugfix and update for AT32F415 (#24807)

[zvecr]

Would we not need to force SEND_STRING_ENABLE = yes when DYNAMIC_KEYMAP_ENABLE is enabled?

[tzarc]

Would we not need to force SEND_STRING_ENABLE = yes when DYNAMIC_KEYMAP_ENABLE is enabled?

Was unaware it could be turned on or off, tbh. Have added a make rule to do exactly as you've suggested.