squash! Add syntax support for toolchain directive

Update Re following https://pkg.go.dev/golang.org/x/[email protected]/modfile#ToolchainRE

Author: matthewhughes934

autoload/go/highlight_test.vim

@@ -680,11 +680,14 @@ function! Test_gomodToolchainVersion_highlight() abort
     let l:dir = gotest#write_file('gomodtest/go.mod', [
           \ 'module github.com/fatih/vim-go',
           \ '',
+          \ 'toolchain default',
+          \ 'toolchain go1',
+          \ 'toolchain go1',
           \ 'toolchain go1.21',
           \ 'toolchain go1.21rc3',
           \ 'toolchain go1.21',
           \ 'toolchain go1.21.3-somesuffix',
-          \ 'toolchain go1.21rc4-somesuffix more suffix',
+          \ 'toolchain go1.21rc2-somesuffix',
           \ ''])
 
     let l:lineno = 3
@@ -734,11 +737,12 @@ function! Test_gomodToolchainVersion_invalid_highlight() abort
     let l:dir = gotest#write_file('gomodtest/go.mod', [
           \ 'module github.com/fatih/vim-go',
           \ '',
-          \ 'toolchain go1.21.1rc2',
-          \ 'toolchain go1.21abc',
+          \ 'toolchain go2',
+          \ 'toolchain go1.21.1.4',
+          \ 'toolchain go1.21.1blah',
           \ 'toolchain go1.21!some-suffix',
-          \ 'toolchain 1.21rc2',
-          \ 'toolchain 1.21.3',
+          \ 'toolchain go1.21 some-suffix',
+          \ 'toolchain something-else',
           \ ''])
 
     let l:lineno = 3

syntax/gomod.vim

@@ -41,21 +41,23 @@ syntax region gomodComment  start="//" end="$" contains=@Spell
 highlight default link gomodComment Comment
 
 " make sure quoted import paths are higlighted
-syntax region gomodString start=+"+ skip=+\\\\\|\\"+ end=+"+ 
-highlight default link gomodString  String 
+syntax region gomodString start=+"+ skip=+\\\\\|\\"+ end=+"+
+highlight default link gomodString  String
 
 " replace operator is in the form of '=>'
 syntax match gomodReplaceOperator "\v\=\>"
 highlight default link gomodReplaceOperator Operator
 
-" match toolchain versions, per https://go.dev/doc/toolchain#version, e.g.
-"
+" match toolchain versions, based on https://go.dev/doc/toolchain#version,
+" and https://pkg.go.dev/golang.org/x/[email protected]/modfile#ToolchainRE
+syntax match gomodToolchainVersion "default$" contained
+syntax match gomodToolchainVersion "go1$" contained
 "  * go1.X.Y
 "  * go1.X
 "  * go1.XrcN
+"  * go1.XrcN-somesuffix
 "  * go1.X.Y-somesuffix
-"  * go1.XrcN-somesuffix more stuff
-syntax match gomodToolchainVersion "go1\.\d\+\(\(.\d\+\)\|\(rc\d\+\)\)\?\([ \t-].*\)\?" contained
+syntax match gomodToolchainVersion "go1\(\.\d\+\)\{,2\}\(rc\d\+\)\?\(-\([a-zA-Z0-9]\+\)\)\?" contained
 highlight default link gomodToolchainVersion Identifier
 
 " match go versions