Feature: Change File Search to use mfind

[tanishq-dubey]

This PR changes the standard file search to use the mfind utilty on MacOS.

mfind uses the OS build Spotlight index to make searching faster, and bias results with usage and recency statistics. This implementation calls out to mfind and streams results back. Because this uses a prebuilt index, serach speeds are stable even as search scope increases. This means we can change the default search path to the entire home directory.

It should be noted that this implementation should be replaced with a native call to NSMetadataQuery or similar in the future to avoid the shell call.

Below is a benchmark comparing the previous method and the mfind method across ~/Documents + ~/Desktop + ~/Downloads and ~, demonstrating stable speeds as scope increases:

============================================
  BENCHMARK: ~/Documents + ~/Desktop + ~/Downloads
============================================

=== WalkBuilder file search benchmark ===
Dirs: ["~/Documents", "~/Desktop", "~/Downloads"]  |  Iterations per query: 5
Query          Min (ms)   Max (ms)   Avg (ms)  Results
--------------------------------------------------------
pdf               14.46      15.89      14.90       33
png               13.94      14.45      14.11      323
doc               13.83      14.49      14.10        9
screen            14.05      14.57      14.29       18
note              14.26      15.49      14.83        4

=== mdfind file search benchmark ===
Dirs: ["~/Documents", "~/Desktop", "~/Downloads"]  |  Iterations per query: 5
Query          Min (ms)   Max (ms)   Avg (ms)  Results
--------------------------------------------------------
pdf               25.34      26.20      25.74       30
png               28.91      32.98      30.50      314
doc               24.39      26.15      25.26        8
screen            25.09      26.62      25.98       20
note              23.88      24.92      24.51        5

============================================
  BENCHMARK: ~ (entire home directory)
============================================

=== WalkBuilder file search benchmark ===
Dirs: ["~"]  |  Iterations per query: 5
Query          Min (ms)   Max (ms)   Avg (ms)  Results
--------------------------------------------------------
pdf            11705.89   17497.28   13041.27      168
png              365.70    1199.77     533.60      400
doc              172.10     186.17     175.55      400
screen         10696.30   11556.99   11344.42      172
note           11461.39   12046.75   11694.27      245

=== mdfind file search benchmark ===
Dirs: ["~"]  |  Iterations per query: 5
Query          Min (ms)   Max (ms)   Avg (ms)  Results
--------------------------------------------------------
pdf               31.99      57.67      37.98      205
png              211.85     481.31     268.02      400
doc               50.02     221.23      85.31      400
screen            31.30      51.75      36.66      147
note              31.64      48.37      35.30      173

[unsecretised]

It should be noted that this implementation should be replaced with a native call to NSMetadataQuery or similar in the future to avoid the shell call.

Would it be possible to do this implementation now itself?