To plant an entire field, hold down the modifier key (default Left Shift) while planting a seed or placing fertilizer, and the mod will plant outward from the point of origin until:
a) it runs out of contiguous space, or
b) you run out of seeds / fertilizer.
By default, contiguous space includes diagonal tiles. You can exclude diagonal tiles in the config.
To plant in a straight line, hold down the straight mod key (default Left Ctrl) instead.
To plant in a wide (3 tiles) straight line, hold down the straight mod key and the main modifier key.
To plant in a 3x3 square around the player, hold down the sprinkler mod key (default Left Alt).
To plant in a 5x5 wide square around the player, hold down the sprinkler mod key and the main modifier key.
When planting in a square around the player, if you don't want the square under the player planted, make sure the mouse is not hovered over the player's tile.
It also works with connected garden pots.
You can edit config values using Generic Mod Config Menu, or editing the config.json file.
Technical
Requires SDV 1.5.5+
Requires SMAPI, uses Harmony.
Compatible with Mod Updater for automatic updates.
Code is at https://github.com/aedenthorn/StardewValleyMods.
If you want to complain or ask for help or help me test my mods, you can visit my Discord server.
A list of all my mods for Stardew Valley is available at https://www.nexusmods.com/stardewvalley/articles/895.